Here are my favorite Open Source apps for OS X, the ones I would install imediately after a fresh OS X installation:
- iTerm terminal emulator.
- Chicken of the VNC, VNC client
- Seashore, GIMP-based OS X app for image manipulation.
- D-Subtitler (extract subtitles from DVD), D-Vision (GUI for mencoder/mplayer) and D-Volution (encode .mov and .dv files in Divx/Xvid or h264) from ObjectifMac
- NeoOffice, a port of OpenOffice.org to the OS X
- Fire an IM client supporting ICQ, MSN, Y!IM, etc..
- MPlayer OS X
- Cyberduck a nice ftp/sftp client.
Of course, if you are into UNIX/Linux you will feel much more at home by installing Darwin Ports...