As part of the work I've started doing on Stateless Linux, I've found myself hacking on device-mapper's snapshot support. It's actually been really good fun to do a bit of kernel hacking again after such a long time ... and this time I feel like I actually have some clue what I'm doing.
I'm really behind the times when it comes to all the new fangled revision control systems out there like bzr, git, mercurial, svn etc. - cvs and manual patch mangling is all I know.
So, when agk pointed me at quilt, I was a bit dubious at first but gave it a shot. It turned out to be a really simple and useful way of maintaining your own patchset in parallel with an upstream codebase.
Even though it's a really simple tool, it seems to me to have some of the important advantages of other systems like distributed offline branches and change sets. If I had time to hack on GNOME again, I'd use quilt a lot for offline branches alongside CVS. It's given me an appetite for looking at other systems too.