The purpose of this page is tracking and explaining the features added in every release, just like the [http://wiki.dragonflybsd.org/index.php/DragonFly_Status Dragonfly people did].
It'd be nice if kernel hackers would spend some minutes adding their stuff here. The one place where you can find a comparable changelog are the fabulous LWN kernel articles: http://lwn.net/Kernel/, or the [http://lwn.net/Articles/driver-porting/ driver porting guide] - but there's no reason why the kernel community shouldn't embrace and extend those efforts 
TODO:
Import relevant data from [http://www.codemonkey.org.uk/post-halloween-2.5.txt davej's post halloween document]
- Keep track of what gets changed
Import [http://kernelnewbies.org/status/latest.html Kernelnewbies status] list (done - some "post-2.6.0" stuff perhaps)
2.6.14:
2.6.13:
- x86 now uses the generic PCI bus setup code for assigning unassigned resources
[http://lwn.net/Articles/104343/ inotify]
Support for the Xtensa architecture: [http://www.tensilica.com/products/xtensa_architecture.htm 32-bit architecture] used in embedded devices
[http://lwn.net/Articles/108595/ kexec and kdump]: Kexec allows users to load a new kernel from another running kernel. By preserving the memory contents in a crash scenario, kexec allows to implement kdump. Kdump is able to get a memory dump of the previous kernel, and be used as a debugging tool.
[http://lwn.net/Articles/135472/ Execute-in-place support]: Traditionally, programs are loaded from disk to memory to be executed. However, the current wave of embedded devices store programs in a ROM/flash chip. XIP allows the kernel executing programs directly from that ROM, without being copied to RAM (saving RAM space), and bypassing the page cache/io scheduler layers (since they're not needed).
[http://lwn.net/Articles/145973/ build-time configurable clock interrupt frequency]:
[http://lwn.net/Articles/143474/ Improved CFQ IO scheduler]
- Voluntary preemption patches
- Removal of the devfs configuration option
2.6.12:
[http://lwn.net/Articles/140164/ API changes]
- New driver for the "trusted computing" (TPM) crap^Wchip
[http://www.superh.com/products/shyway.htm SuperHyway bus support]
- Multilevel security implementation for SELinux
[http://lwn.net/Articles/124703/ device mapper multipath support]
[http://lwn.net/Articles/121845/ Address space randomization]
- Restore the Philips webcam driver
- I/O barrier support for serial ATA drives
[http://lwn.net/Articles/134460/ "resource limits"]
[http://lwn.net/Articles/127936/ cpusets]
- Remove IPV6 "experimental" status
2.6.11:
2.6.10:
2.6.9:
2.6.8:
2.6.7:
2.6.6:
[http://lwn.net/Articles/80472/ full object-based reverse-mapping scheme and removal of the per-page PTE chains]
- Network packet timestamping optimization
[http://www.geocities.com/wronski12/posix_ipc/index.html POSIX message queues]
- Message queues for the x86_64 and s390 architectures
- fsync() and fdatasync() speed improvements to ext2/3
[http://lwn.net/Articles/77190/ addition of the fcntl() method to the file_operations structure]
[http://lwn.net/Articles/65437/ "Laptop Mode"]
- 4KB kernel stacks option for the i386 architecture
- Non-executable stack support for several architectures
- Reiserfs updates: data=ordered support, space preallocation, laptop mode support
- IPv6 support in SELinux
[http://lwn.net/Articles/79326/ The lightweight auditing framework]
- A mechanism which allows block drivers to respond to queries about the congestion state of their queues
[http://lwn.net/Articles/75233/ "per-device unplugging patch"]
- CFQ scheduler
- External module support
2.6.5:
- Netpoll infrastructure
2.6.4:
- HFS rewrite
2.6.3:
2.6.2:
2.6.1:
2.6.0:
2.6.0-test9:
[http://lwn.net/Articles/44243/ libata driver architecture]
2.6.0-test6:
2.6.0-test3:
[http://www.nsa.gov/selinux/index.html SELinux]
2.6.0-test1:
[http://www.linuxvirtualserver.org/ Linux Virtual Server layer]
2.5.75:
[http://marc.theaimsgroup.com/?l=linux-mm&m=104529418208788&w=2 Anticipatory Scheduler]
2.5.74:
2.5.73:
2.5.72:
2.5.71:
[http://marc.theaimsgroup.com/?l=linux-kernel&m=104981044405395&w=2 Switch the IDE I/O layers to taskfile]
2.5.70:
[http://www.linux-usb.org/gadget USB gadget support]
2.5.69:
[http://lwn.net/Articles/29555/ New interrupt handling API]
2.5.68:
2.5.67:
2.5.66:
2.5.65:
[http://www.kerneltrap.org/node.php?id=603 Desktop Interactivity Improvements]
2.5.64:
2.5.63:
[http://high-res-timers.sourceforge.net/ POSIX timers]
2.5.62:
[http://lse.sourceforge.net/locking/dcache/dcache_lock.html Avoid dcache_lock while path walking]
2.5.61:
2.5.60:
- New modversions implementation
- 64-bit jiffies
2.5.59:
[http://home.arcor.de/efocht/sched/ NUMA aware scheduler extensions]
2.5.68:
2.5.57:
[http://www.zipworld.com.au/~akpm/linux/schedlat.html Remove long-held locks for low scheduling latency]
2.5.56:
2.5.55:
2.5.54:
[http://secure.netroedge.com/~lm78/ Add lm-sensors drivers for hardware health monitoring]
- Support for AGP 3.0
2.5.53:
- "sysenter" support
2.5.52:
2.5.51:
[http://linuxconsole.sourceforge.net/ Rewrite of the console layer]
- Compatibility syscall layer
2.5.50:
2.5.49:
2.5.48:
[http://lists.insecure.org/lists/linux-kernel/2002/Oct/6214.html In-kernel module loader]
2.5.47:
2.5.46:
[http://www.xmailserver.org/linux-patches/nio-improve.html Better I/O performance with epoll]
Per-cpu hot & cold page lists
[http://lists.insecure.org/lists/linux-kernel/2002/Oct/7027.html MMU-less processor support (ucLinux)]
[http://marc.theaimsgroup.com/?l=linux-kernel&m=101095500820185&w=2 Replace initrd by initramfs]
- Extended Attributes and ACLs for ext2/ext3
2.5.45:
[http://www.xs4all.nl/~zippel/lc/ New Linux configuration system: kconfig]
[http://samba.org/~jamesm/crypto/ CryptoAPI]
[http://www.linuxtv.org/dvb/ Digital Video Broadcasting (DVB) layer]
[http://www.sistina.com/products_lvm.htm Device mapper for Logical Volume Manager (LVM2)]
2.5.44:
- New sysfs filesystem (formerly known as driverfs)
- Plug'N Play Layer Rewrite
[http://lwn.net/Articles/9042/ x86 BIOS Enhanced Disk Device (EDD) polling]
2.5.43:
[http://www.psc.edu/general/filesys/afs/ Andrew File System (AFS) support]
[http://oprofile.sourceforge.net/ OProfile, a low-overhead profiler]
[http://lse.sourceforge.net/locking Read-Copy Update (RCU) Mutual Exclusion]
[http://www.citi.umich.edu/projects/nfsv4/ Add support for NFS v4]
- Remove kiobufs
2.5.42:
[http://marc.theaimsgroup.com/?l=linux-kernel&m=103298446916066&w=2 ext2/ext3 large directory support: HTree index]
[http://us1.samba.org/samba/Linux_CIFS_client.html Add new CIFS (Common Internet File System)]
[http://www.gelato.unsw.edu.au/ Remove the 2TB block device limit]
- Improved i2o (Intelligent Input/Ouput) layer
2.5.41:
2.5.40:
[http://lse.sourceforge.net/numa Parallelizing page replacement]
[http://lse.sourceforge.net/numa NUMA topology support
[http://www.brodo.de/cpufreq/ Add support for CPU clock/voltage scaling]
2.5.39:
- New IO scheduler ("deadline")
2.5.38:
2.5.37:
[http://users.pandora.be/bart.de.schuymer/ebtables/ Ethernet bridge tables support]
- Remove the global tasklist
2.5.36:
- Hugepage support
[http://oss.sgi.com/projects/xfs/ XFS filesystem from SGI]
2.5.35:
- Serial ATA support
[http://user-mode-linux.sourceforge.net/index.html Add User-Mode Linux (UML)]
2.5.34:
- POSIX threading support for signals
[http://lse.sourceforge.net/numa discontigmem support (ia32)]
2.5.33:
- TCP segmentation offload
[http://www.sf.net/projects/lksctp SCTP (Stream Control Transmission Protocol)]
2.5.32:
[http://people.redhat.com/drepper/nptl-design.pdf Improved POSIX threading support]
[http://freshmeat.net/projects/linux-aio/ Asynchronous IO (aio) support]
- Porting all input devices over to input API
- New MTRR (Memory Type Range Register) driver
2.5.31:
- Support insane number of processes
- Disk description cleanups
- Remove incomplete SPX network stack
2.5.30:
- Remove khttpd
2.5.29:
- Strict address space accounting
[http://lsm.immunix.org/ Add Linux Security Module (LSM)]
[http://people.redhat.com/mingo/tls-patches/ Thread-Local Storage (TLS) support]
2.5.28:
- Remove the "Big IRQ lock"
- Serial driver restructure
2.5.27:
[http://surriel.com/patches/ New VM with reverse mappings]
2.5.26:
Direct pagecache <-> BIO disk I/O
2.5.25:
[http://www.uwsg.iu.edu/hypermail/linux/kernel/0207.0/0741.html Faster internal kernel clock frequency]
- Unified naming for disk devices
2.5.24:
2.5.23:
[http://www.kernel.org/pub/linux/kernel/people/rusty/patches/Hotcpu/ Hotplug CPU support]
[http://bazar.conectiva.com.br/~acme/patches/wip More complete IEEE 802.2 stack]
2.5.22:
2.5.21:
2.5.20:
2.5.19:
2.5.18:
[http://falcon.sch.bme.hu/~seasons/linux/swsusp.html Software suspend (to disk & RAM)]
->getattr() ->setattr() ->permission() changes
2.5.17:
- Move ISDN4Linux to CAPI based interface
[http://marc.theaimsgroup.com/?l=linux-fsdevel&m=101586205801483&w=2 New quota system supporting plugins]
2.5.16:
2.5.15:
2.5.14:
- Bluetooth support (no longer experimental!)
[http://www.uwsg.iu.edu/hypermail/linux/kernel/0204.1/1250.html Support for IDE TCQ (Tagged Command Queueing)]
2.5.13:
2.5.12:
- Rewrite of the buffer layer
2.5.11:
- Rewrite of the framebuffer layer
[http://www.ussg.iu.edu/hypermail/linux/kernel/0204.3/0129.html Fast walk dcache]
[http://linux-ntfs.sourceforge.net/ Replace old NTFS driver with NTFS TNG driver]
2.5.10:
2.5.9:
- Smarter IRQ balancing
2.5.8:
[http://www.uwsg.iu.edu/hypermail/linux/kernel/0204.1/0215.html Radix-tree pagecache]
[http://www.kernel.org/pub/linux/kernel/people/rml/cpu-affinity Syscall interface for CPU task affinity]
2.5.7:
[http://sourceforge.net/projects/acpi/ ACPI (Advanced Configuration & Power Interface)]
[ftp://robur.slu.se/pub/Linux/net-development/NAPI/ NAPI network interrupt mitigation]
[http://www.uwsg.iu.edu/hypermail/linux/kernel/0203.0/0884.html Futexes (Fast Lightweight Userspace Semaphores]
[http://www.uwsg.iu.edu/hypermail/linux/kernel/0203.0/0335.html Video for Linux (V4L) redesign]
[http://www.hpl.hp.com/personal/Jean_Tourrilhes/Linux/Tools.html New driver API for Wireless Extensions]
2.5.6:
[http://www.hojdpunkten.ac.se/054/samba/ http://www.hojdpunkten.ac.se/054/samba/]
[http://www.cs.helsinki.fi/linux/linux-kernel/2002-01/0438.html HDLC (High-level Data Link Control) update]
[http://www.uwsg.iu.edu/hypermail/linux/kernel/0202.2/0920.html per_cpu infrastructure]
[http://www-124.ibm.com/jfs/ Add JFS (Journaling FileSystem from IBM)]
- Killing kdev_t for block devices
2.5.5:
[http://linuxppc64.org/ New architecture: PowerPC 64-bit (ppc64)]
[http://www.x86-64.org/ New architecture: AMD 64-bit (x86-64)]
[http://www.uwsg.iu.edu/hypermail/linux/kernel/0202.2/0297.html Pagetables in highmem support]
[http://www.alsa-project.org/ Add ALSA (Advanced Linux Sound Architecture]
2.5.4:
[http://www-124.ibm.com/developerworks/oss/pthreads/ Support for Next Generation POSIX Threading]
[ftp://ftp.kernel.org/pub/linux/kernel/people/rml/preempt-kernel/ Add preempt kernel option]
Per network protocol slabcache & sock.h
[http://marc.theaimsgroup.com/?l=linux-kernel&m=100942323913586&w=2 Per filesystem slabcache & fs.h]
2.5.3:
[http://kernel.org/pub/linux/kernel/people/mochel/doc/ New driver model & unified device tree]
[http://www.uwsg.iu.edu/hypermail/linux/kernel/0109.3/1341.html PnP BIOS driver]
[http://www.geocrawler.com/mail/msg.php3?msg_id=7686796&list=35 Generic Extended Attribute support]
[http://marc.theaimsgroup.com/?l=linux-kernel&m=101077476005939&w=2 Support reiserfs external journal]
[http://marc.theaimsgroup.com/?l=linux-kernel&m=101121711408999&w=2 IDE layer update]
[http://www.uwsg.iu.edu/hypermail/linux/kernel/0202.0/0260.html Untangle sched.h & fs.h include dependancies]
[http://www.geocrawler.com/lists/3/Linux/35/100/7642826/ Break Configure.help into multiple files]
2.5.2:
[http://marc.theaimsgroup.com/?l=linux-kernel&m=100992347220716&w=2 New kernel device structure (kdev_t)]
[http://people.redhat.com/mingo/O(1)-scheduler/ O(1) scheduler for improved scalability]
[http://marc.theaimsgroup.com/?l=linux-fsdevel&m=98307457705210&w=2 Filesystem per-process namespaces, late-boot cleanups]
[http://www.linux-usb.org/usb2.html Initial support for USB 2.0]
2.5.1:
- Rewrite of the block IO (bio) layer