I see there are a few really good sites listing some excellent open source OS X applications, but here are a few that they always leave out that I think deserve to be included.

VirtualBox - Run another OS inside OS X, like Windows or Linux.

NEO Office - Full Office solution built off of Open Office but natively conpiled for OS X.

Traverso - Professional DAW audio recording.

FScript - Apple script editor.

XAMPP - Self contained Apache web server with SSL, database, PHP, mod_perl, etc. ready to roll.

Bean - Powerful word processor, text editor. Lots of great features...

Cord - Window's Terminal Server Client.

Handbrake - Video transcoder. 

RSS Owl - News reader.

Wormux - Really fun network game of team warfare!