Internet related:
- Adium
The multi-protocol instant messaging client for the Mac. Handy if you have MSN, ICQ, and Google Talk accounts (like me). - GlimmerBlocker
The only adblocker for Safari that isn't implemented as a hack -- this one is actually a proxy that filters out the stuff you don't want to see. - Transmission
Simply the best torrent client for OS X. - Google Notifier + Google + Growl
A menu bar app that notifies you when there's new mail in you Gmail inbox. The Google + Growl utility makes sure the notifications are Growl compliant.
LaTeX:
System tools:
- Growl
A notification system for Mac OS X. Many programs are capable of using it, and it's a functionality that's lacking by default in OS X. - USB Overdrive
The default mouse acceleration is really crappy on OS X. USB Overdrive lets you change it according to your own tastes. - CDto
CDto adds a button to Finder that opens a Terminal window and changes its active directory to the Finder directory.
- Plex
Plex is a home theatre app that is much more versatile than Front Row. Amongst other things, it can pull content directly from the internet to your TV. - Perian
Perian adds playback support to QuickTime for a whole range of media format. - Flip4Mac
Adds WMV support to QuickTime. - ScrobblePod
If you've got a Last.FM account, this little app is for you. It scrobbles all your plays in iTunes.
Other:
- Jin
Jin is the only chess client that runs on a Mac and supports the Free Internet Chess Server. So if you're a cheap bastard like me and enjoy a game of chess, Jin is the way to go.