Open Source Software I Appreciate
I use all of these programs on my mac. Many of them are also available for linux and/or windows.

Bibdesk: Front-end for keeping track of bibtex entries. Extremely useful and user friendly.
Cyberduck: FTP/SFTP program for file transfer.
Gimp: Open-source equivalent of photoshop.
Mactex: Latex package for mac.
SeaMonkey: Web browser and easy-to-use HTML editor.
Unison: A great free utility for backing-up and syncing. Apparently is no longer under active development.