First of all, sorry for reviving an old thread, but here's a must have App list I've compiled for OS X:
Free
-Firefox
-uTorrent/Transmission
-Adium
-VLC
-PS3 Media Server
-Magnifique
-Onyx
-The Unarchiver
-TinkerTool, extended preferences
-Growl, notifier app, for almost any other app
-Cyberduck, SSH/SFTP client
-SuperDocker, Change the appearance of your Dock, some useful System settings
-LameSecure, to lock your folders with a password
-Picasa
-Cameras, for handling multiple cameras
-DateLine, minimalistic calendar on you OS X desktop
-GeekTool, for the geek in you !
-HandBrake, great video encoding tool
Paid
-Tweetie, if you use Twitter regularly
-Toast Titanium
-CandyBar, for changing app icons
-AppZapper, to uninstall apps safely
-iWork '09, coz its way better than Office Mac 2008
-TechTool Pro 5
-World Of Goo, insanely addictive game !
-DivX Pro, to enable DivX in Front Row
-Rowmote, app for iPhone, to control Front Row
-Diablo 2, FTW !
-Pixelmator, excellent Photoshop replacement
-StellarPheonix, data recovery tool
-Silk, to change UI font
-CrossOver, to run some Windows apps
-Visage Login, for tweaking your login screen
-DragThing
-DropZone, extends the drag-and-drop function to a awesomely high level
Firefox Extensions
-DownThemAll, download manager
-Adblock Plus
-Mozilla Weave/Xmarks, syncs your bookmarks to a cloud
-GrApple Delicious, theme to match the Leopard look