coverflow is fun - a cool way of looking through your itunes library
VLC - for playing .wmv/.avi files
Google Earth - look at your house from satellite images!
Handbrake - a good tool for Ripping DVDs
Delicious Library - fun, but totally useless app, it uses your iSight, you hold up any ISBN bar code to it, and it goes to the internet to see what it is, games, books, etc and archives it for you, really cool.
Google Sketchup - 3D modeling tool, very awesome and useful
iAlertU - alarm that utilizes the motion sensors in your 'book to detect movement and trigger an alarm (flashing screen siren etc.)
OurTunes - a good way to copy music over a network from anyone's iTunes your connected to, you know, their shared music. (this comes in handy for college,

)
Transmission - for bittorrenting
TinkerTool - cool customization tool
CandyBar - if you want to change any Icons, this is the way to do it.
BootCamp Beta - i think you know about it, but just in case, there it is