This is probably the most awesome app ever conceived. Contains most of the functionality of LaunchBar and Quicksilver + several other apps too, all with a tiny memory footprint.
If you like to keep an eye on CPU use, bandwidth, mem usage, this will give you the readouts. Essential on every working person's Mac.
Fast, slick browser, and very functional too. Based on the open source Gecko core, but has the OS X appeal.
The swiss army knife of video players. Can play almost everything except new WMVs. I use this instead of Quicktime Player 99% of the time. Required.
Almost all of the features of Photoshop at an infinitely smaller price. Based on the GIMP base, but with a more Photoshop-like interface. Requires X11 but you can find that on your OS X install disc. And they just released a Universal Binary too.
Entirely free Gnutella/LimeWire network P2P client. Can't find a copy of that old 60's song you love? This will find it. Cross-platform as well.