The nice thing about Apple is a lot of stuff is included out of the box, and Apple Update will make sure you have the latest, so I don't think it makes a lot of sense to list stuff like Safari or iTunes..
I'll throw down a few of my faves...
audio:
Audio Hijack
http://www.rogueamoeba.com/audiohijack/
blogging:
Marsedit
http://ranchero.com/marsedit/
anal organization:
Delicious Library
http://www.delicious-monster.com/index.php
viewing Xvids
VLC
http://www.videolan.org/vlc/download-macosx.html
Mplayer OSX (not as good as VLC, but still useful)
http://mplayerosx.sourceforge.net/
bittorrent
Azureus (great RSS feed grabbing(
http://azureus.sourceforge.net/howto_osx.php
Quicksilver (keyboard driving navigation, use it all the time)
http://quicksilver.blacktree.com/