Linux 2.5.35 (2.5)
http://freshmeat.net/releases/97381/
Linux is a clone of the Unix kernel, written from scratch by Linus
Torvalds with assistance from a loosely-knit team of hackers across the
Net. It aims towards POSIX and Single UNIX Specification compliance. It
has all the features you would expect in a modern fully-fledged Unix
kernel, including true multitasking, virtual memory, shared libraries,
demand loading, shared copy-on-write executables, proper memory
management, and TCP/IP networking.