Well, I'll mostly be seconding apps already mentioned but:
SubEthaEdit (V. 2.2 is still available for free)
Firefox (More specifically, Firefox with Adblock, SessionSaver, Flashblock, and ShowIP)
Sizzling Keys (F1 -> Back F2 -> Play/Pause F3 -> Forward)
MacTheRipper
VLC (Raw Awesomeness)
Kismac (My Passive Wireless Scanner/Cracker of choice!)
AdiumX (The only other protocol I wish it supported is IRC...)
Clutter (I only wish I could use the default artwork it creates when it can't find anything...)
coconutBattery (I'm not sure why I like to know that my battery can now only hold 1017 mAh, which will no doubt go down to 800 next time I run it down)
DesktopManager (It could be smoother, but I couldn't do it better... Virtual Desktops are a dream come true (now make them native in 10.5 Apple!))