evernote: online note syncing and more
adium: multiple chats
dropbox: online file syncing
handbrake: dvd ripper/video encoder
picasa: photo browser and more
chronosync: sync folders through wireless/ethernet
keyboard meastro: hotkeys to apps and more
growl: universal notificactions
sizzling keys: itunes control through hotkeys
last fm: social network costom radio
For me, photography is an important hobby so my top applications revolve around them for dealing with images and producing web pages for said images.
Vmware Fusion (running fedora 12)
AppCleaner; deletes all an application's associated files that wouldn't otherwise be removed if the application was deleted with the trash. Perian; adds support for additional media types in Quicktime. Flip4Mac; adds support for WMV/WMA files to be played in Quicktime.