I can't believe nobody has mentioned
Quicksilver yet, the perfect compliment to Spotlight.
On top of that I'm also a fan of the following (I'll try to mention the ones that haven't been mentioned yet):
Synergy: a nice hotkey/menubar iTunes controller
Ejector: menubar utility for ejecting disks, pretty handy.
iPhoto Buddy: allows you to create multiple iPhoto libraries, invaluable if you have lots of photos.
OmniGraffle: a beautiful tool to help you create charts, flow diagrams, map out your ideas etc.
MailStamps: to change the much talked about Mail toolbar icons to something more pleasing.
PodWorks: move your music from your iPod to iTunes.
Sidenote: newly discovered app that has become party of my daily work flow already. Think perfectly implemented Stickies.
Tomato Torrent: for your legal bittorrent needs.
VLC: not yet perfect on Tiger but still good enough, handles all the files that Quicktime doesn't.
There's already a couple of best freeware/shareware threads about, which you might want to search for. But I feel it's good time to start another one, with the introduction of Tiger and all.
Oh and since everybody is touting VersionTracker, I'm going to throw
MacUpdate.com into the ring. Like VersionTracker but cleaner and Mac centered, which makes it worthy of support in my book
