Have patches to take out non-free stuff, (Debian Free Software Guide in regards to what non-free is).

Some also add support for problem hardware, some have a plethora of patches they maintain until upstream (Linus' Kernel) integrates or "fixes" the problems or bugs or what ever. That is Debian, one of the largest give-back distros. As Greg Kroah-Hartman is the Gentoo kernel Maintainer and a Core Developer for the Official Tree contributions to be integrated into Linus' BK Tree, He boasts that there are only 3-5 patches at any one time in the "newest" Kernel package as compared to the Reference Vanilla Kernel. Gentoo also has the best "build" testing know to man... because everyone builds in Gentoo (well almost, as there are prebuilts)

Personally, some of the Esoteric patches RedHat has had for a number of years, is their own VM for the Kernel. It is still based on the pre-2.4.10 VM that Linus tore out and put a new one in for 2.4.10 at nearly the last minute (in terms of Development timeline that is). <rant>That single patch annoys the SHIITE out of me. It makes the machines Based on RedHat's patch set seem sluggish. Well, not really sluggish, but not as quick-feeling as I'd like. It like the old Rochester Quadra-Jet. It always works when clean and properly maintained, except for when the springs controlling vacuumm response to fuel flow get weak... there is a bit of stutter when you goto WOT and sometimes it back-fires once or twice *THEN* takes off like all hells fury. The one thing this patch changes is the response to HEAVY load. It doesn't start wailing on itself like the "new stock" VM does, flailing memory in and out of disk swap, scouring the Memory for free pages to scrub, basically a machine that is too small for the job intended. If you have a machine that is more than you need, the VM takes performance away from you. Leaving about 3%-15% (depends on the arch BTW) is not my idea of a good thing. </rant>

There are other distros that Heavily modify the source, they like to see familiar names in the messages scrolling by. By familiar I mean theirs.