Things I do in Gnome (kind of old, but still pretty relevant):
I'm a member of the bugsquad, and a bugmaster. I've triaged an awful lot of bugs, written large portions of the triage guide (which has been moved to live.gnome.org but has been butchered by sticking everything into a single page; I haven't yet had time to fix it), helped run a few Bug Days in the distant past, and fixed lots of issues and added new features to Gnome bugzilla. I serve as co-maintainer of bugzilla along with the much more capable Olav.
I am also somewhat active with the Gnome Love group, whose goal is to help new people become contributors to the Gnome project. I try to answer questions on the mailing list, I wrote a special report for this group for finding relevant bugs in bugzilla, I try to occassionally bring up tasks that people could work on, and I wrote a tutorial on developing with Gnome that I hope is helpful. I've been slacking on the guide and haven't updated it in years, but I'll get back to it at some point.
I'm also a member of the Release Team (and technically the release team manager now, but I'm trying to make that not mean much, especially since all the other people on the team are more qualified and smarter than me). We do a variety of things to make sure that releases run smoothly. This ranges from answering questions of module maintainers to helping enforce freezes (and approving breaks) to finding other ways to make things run more smoothly. There are general tasks that everyone does as well as individual tasks that tend to get divided up; in the latter category I've tried to organize or provide bug list overviews, updated documentation, made releases, lent a hand at fixing up some important showstopper issues, worked on improving and making new scripts to ease the release process, and various other tasks that I'm probably forgetting at the moment.
I'm a maintainer of Metacity (along with Soeren and Havoc). I originally concentrated on fixing focus issues and later constraints issues but I've fixed a lot of other random issues too. The cool thing about hacking on Metacity is that sometimes it means getting toolkit and application support for various features. So while trying to add Metacity features, I've ended up hacking on gtk+, gnome-desktop, libwnck, gnome-panel, startup-notification, gnome-terminal, gnome-applets, gnome-utils, nautilus, gok, galeon, and epiphany. Another cool thing about Metacity is that window managers seem to be an area where we've got really good cross-desktop compatibility and there's lots of discussion about specifications (mainly in the form of the EWMH) to ensure this compatibility continues and increases (though the less cool thing is that I've made a fool of myself multiple times in such discussions).
My work on libwnck has lead to me becoming a co-maintainer for it (along with Vincent, Mark, and Havoc). I focus my time more on Metacity but I still try to help keep things under control for libwnck. Supposedly I'm a co-maintainer for the startup-notification module in freedesktop.org cvs as well, or at least Mark listed me as such on the Desktop modules listing page but I'm unlikely to touch that module again unless something special is added to the EWMH that requires it.