While I tend to write small patches for bugs I encounter in pretty much any application I use that is open source, there are several projects I contribute to on a regular basis.

Samba

The official Samba homepage.

I've been a member of the global Samba team for some years now, and am currently mostly working on the next version, Samba 4.

My homepage at Samba.org has more details.

BitlBee

The official Bitlbee homepage.

I am one of the three active BitlBee developers. I maintain my own branches of Bitlbee that contain my changes that haven't been merged into the release tree yet by Wilmer. They are available in bzr at http://code.bitlbee.org/jelmer/.

I also maintain the Windows port of BitlBee.

wireshark

The official wireshark homepage.

I occasionally contribute protocol dissectors and I maintain the wireshark dissector generator of pidl (the Samba4 IDL compiler).