On my macbook:
Browser: Safari 5
Email: Apple Mail
Coding: Komodo IDE (local), Coda (remote)
FTP/SFTP: YummyFTP
Photo Editing: Photoshop CS5
Virtualization: Vmware Fusion 3.1
IRC: Colloquy
IM(aim, msn, icq, y!, gtalk, etc): AdiumX
Music: iTunes (sucks on windows though)
Text Editor, raw code editor: Textmate (second favorite: TextWrangler)
Remote File storage and sharing: DropBox
Video encoding: Handbrake
Disk defragmentation (installed on external bootable OSX drive): iDefrag
Network Security: Little Snitch
SSH/Terminal: Terminal
MySQL GUI (when needed): Navicat Pro
OSX Tweaks and Hacks: Onyx
Video Playback (once Perian and Xiph have been installed): QuicktimeX and VLC
Drive/Disc copying: Super Duper
Bittorrent : Transmission (or uTorrent Beta)
ToDo/Organization: Things
UnRar : UnrarX
Drive/Thumbdrive encryption: TrueCrypt
Boredom Games: Plants vs Zombies, Half Life 2, Peggles
Some linux stuff can be used on OSX if you install X11 from the DVD (ie: install Xcode)