OpenBSD 3.5 and Debian stable with a custom 2.4.26 kernel
(2.6 still feels to new to me, at any rate. theres no reason to more onto it.)
since i figure you havent heard of openbsd before:
The OpenBSD project produces a FREE, multi-platform 4.4BSD-based UNIX-like operating system.
and debian...is a linux distro. im sure youve heard of linux.
their both very highly respected operating systems. i didnt come to end up using them by chance - or because they came installed