quicksilver
with email to... (send directly)
(this one piece of software has improved my life immeasurably)
isight
with dailymugshot.com
pandorajam
with last.fm scrobbling
textmate
with rails support
spaces
the best thing since cmd/alt+tab
exposé
with spaces hotkey support
things
with iphone/ipod touch app syncing
google notifier
growl
istumbler
dashboard
with 1337pwn Friends (xbox live status), iStat nano, TV Show Tracker
dock
with stacks (fan view!)
with gmail support, import senders into address book
anything else you think I should check out?