Jitouch - extended gestures for Magic trackpad and Magic mouse
Sparrow - native client for Gmail
Dropbox - no further comments needed, I think
Mail Unread Menu - unread mail count for Mac mail client - with custom icons
- communicator
Adium - communicator
Little Snitch - Internet connection rules for apps
Blast - frequently used documents and apps menu
Meerkat - SSH tunneling to my home NAS server
Sidekick - activates apps and settings based on my location (E.g. disables locking computer when I am at home, etc.)
ExpanDrive - mounts remote drive (FTP, SFTP, etc.) - has a plugin for Network location
BusyCalAlarm - latest calendar events
Sophos Antivirus - there are viruses and malware designed for all OSes
Evernote - great way to record notes
Caffeine - great for keeping the screen awake
The rest is just standard Mac stuff.
Here are few things, that I have hidden from the menu bar — there is way too much stuff already:
Alfred - quick access / search anything
TextExpander - replaces texts while typing for quick text typing
Moom - window management tool
Witch - replaces Cmd+Tab with extended features (mainly restores minimized windows when switching)
TotalFinder - extend Finder functionality
Visor - docks terminal Quake style
(From Ondrej Masek)