Stability, thy name is Slackware. The new version 11, just out, is still running the 2.4 kernel, and is so efficient that the official Slackware website still runs on a Pentium III 600MHz with 512MB RAM.
One of the oldest Linux distributions (going on thirteen years now) has hit a new milestone with its latest 11 release, and despite including new versions of all the essentials including KDE, bucks the trend when it comes to the kernel -- 2.4 is the name of the game for Slackware 11, and that's not the only package to err on the side of stability.
It's certainly a refreshing change from bundling the very latest with each new release. Slackware 11 does do this for the essentials -- KDE 3.5.4, XFCE 4.2.3.2, Firefox and Thunderbird 1.5.0.7, and the latest must-haves of Amarok and XChat, but the default kernel is 2.4.33.3 based and the X.Org implementation pre-modular 6.9.
Two 2.6 kernels are optionally included however, coming with a 2.6.17 based kernel (the current 'stable' 2.6 branch) and a 2.6.18 'testing' kernel. Gnome as a desktop isn't included, but popular Gnome applications (like Gimp, and XChat) are. GCC and glibc stay at 3.4.6 and 2.3.6 repsectively, again showing a focus on known to be stable tools over the newer (though not necessarily less stable, we're sure it's a matter of debate) GCC 4.1 series or glibc 2.4.
Slackware's stated aim is to '[Include] the latest popular software while retaining a sense of tradition, providing simplicity and ease of use alongside flexibility and power' and has, as a result, developed a strong and loyal userbase -- this Linux geek cut his teeth on Slackware, no less.
And by way of example of its focus, even the official site runs Slackware on a 'Pentium III, 600 MHz, with 512 megabytes of RAM'.
In fact Slackware deserves more recognition than it gets in the populated world of Linux distributions (Distrowatch lists over 500 at last count!) so if you like to experiment with distributions and play with one that's been around since Windows 3.1 (and, yegads, OS/2 even), give Slack a go.
Head to the Slackware homepage, download ISOs from the mirror list or snarf a copy on Bittorent with the CD and DVD image torrents
If you're running a 64-bit platform there are not one but two Slackware 64 ports available: Slamd64, and Bluewhite64.
The minimalist desktop of XFCE. Source: Bluewhite64