Here are some open source projects that have very supportive communities:
WebKit is, among other things, a great open source browser engine. It is used by Safari, Chrome, PalmOS, and many other high impact projects. It is very friendly to new developers. Stop by http://webkit.org/, visit us on irc in #webkit on irc.freenode.net, or email me at jberlin@webkit.org if you are interested in getting involved!