Oh man, I have SO MANY!
Most of them have been said, but I'll reiterate them anyways.
AIM: Adium (bad with file transfers though, use iChat for that)
**Quicksilver is a must!!!!**
Desktop Manager (for having multiple desktops, switch between them with
cool gui shifts... example being you have four desktops, and you rotate
around a cube to show different desktops)
SVN: ZigVersion (still in beta, but it's very simple!)
SQL: CocoaSQL (very quickly and easily view your database from this
Acquisition + Azureus for P2P
Audacity for Audio editing
Chicken of the VNC for WinVNC
FinkCommander for getting your favorite 'nix flavor program ports!
iEatBrainz for automatically entering ID3 tags for MP3s
iLinkPod for grabbing MP3's off of your iPod
MacStumbler for checking Wireless activity (strengths, etc)
Toast 6 Titanium for all things ISO/Burning etc.
***IMPORTANT: VLC for all video playback!***
File merging: OSX has a built in file merger!!! FileMerge (c'mon mac
guys, how'd u miss this one)
Database graphic design: OmniGraffle (so simple and powerful)
CSS Cheat Sheet
Congratulations on switching! You'll never go back! You'll be an
apple zealot soon enough...