Here is a list of useful Mac OS X applications, i will try to keep this list updated:
Perian: Videos codecs for Quicktime
Dropbox: Sync and share files on the internet
Flip4mac: Play WMV video inside Quicktime
Adium: Multi-protocol instant messenger
GPGTools: Encryption tools for Mac OS X
Wunderlist: Manage and share todo lists
Freemind: Create and edit mindmaps
Transmission: Download bittorrent files
AppCleaner: Remove files left behind after deleting applications
AppFresh: Keep your applications up to date
OnyX: Maintenance and optimization tool
Skype: Voice and video conferencing over the internet
Teamviewer: Easily share your desktop for remote access over the internet
Virtualbox: Run Windows and other operating system inside your Mac without rebooting
VLC: Read almost any media file
Handbrake: Convert DVDs to media files
OpenOffice: Text, spreadsheet and poresentation editor
Evom: Convert media files to iTunes format
The Unarchiver: Extract files from any archive format
SousChef: Manage recipes and grocery lists
Skim: Easily annotate PDFs
Xbench: Benchmark utility for your Mac
jDownloader: Easily queue file transfer from popular download services
Winebottler: Create Wine bottles to emulate Windows apps inside Mac OS X
The paid apps:
Pixelmator: Powerfull image editor ($59.99)
1Password: Store all your password in one location ($39.95)
Rivet: Share media with your Xbox 360 and PS3 ($19.95)
Data Rescue: Recover data from failed hard drives ($99)