Here's some of what I use...
BBEdit lite (Free) Its the only text editor I found that can handle Windows/non-Windows carriage returns properly.
Sidetrack (Nagware) to allow me to scroll with my trackpad.
Adium (Free) for IM.
MT-Newswatcher (Free) for newsgroups. Extremely powerful.
Cyberduck (Free) for FTP.
VLC (Free) for media player.
Acqlite (Free) for P2P
iMergePDFs (Free) to merge PDFs.
Chicken Of The VNC (Free) VNC client
X-Chat Aqua for IRC.
Dashboard Starter (Free) to pre-load the Dashboard on startup...
UnRarX (Free) for decompressing files.
Firefox (naturally) & OpenOffice
I did use Tomato Torrent at one point, but removed it. Can't remember why though
I use Azureus atm, but could be worth a try if you want an OSX-native alternative.
The best site i've found for finding new Mac software is:
http://www.versiontracker.com/