Here's a few of the apps I've been using lately:

Synergy (iTunes controller in the menu bar, works with growl)

iScrobbler (works with www.last.fm, basically tracks what you play in iTunes and builds charts based on your listening habits)

MenuCalendarClock iCal (good replacement for the basic clock on the menu bar, you can have it show the time or not, but it adds a pulldown type menu to the menu bar that shows this month's calendar and shows your iCal appointments... basically, you'd only have to use iCal to add something)

RapidWeaver (web page editor, while it has a lack of good themes that come free, the cool thing about this program is that you can switch themes on the fly... something iWeb should consider doing.)

Pages (part of the iWork suite, overall it's a good word processor. It's integrated with the iLife suite, you can drag and drop with this too... and something I liked, it has a template for MLA styling which a lot of papers I've had to write had to be in MLA... plus it's cheaper than getting Microsoft Office)

iSerial Reader (when you absolutely, positively don't want to pay for something)

Backup (comes with .Mac, I use it to backup my Home folder to .Mac... it also offers for you to backup you music collection or anything else you don't want to lose to a cd/dvd or an external drive and you tell it when you want to do this by scheduling it.)

Nicecast(very nice internet streaming program that actually uses iTunes)

Transmit (while I don't use this much, it's the best FTP client on the Mac hands down)

Aurora (turns your Mac into an alarm clock, it wakes the Mac from sleep mode at the time you set and it will play any iTunes playlist you tell it to upon waking up)

Colloquy (good IRC client, best feature is that you can set up filters on curtain bots in an IRC room, good for when things are moving way too fast)

Adium (like someone said before, it's iChat on crack... it's the only client at this time to support chat in Yahoo... only downfall is it doesn't offer voice or video... but it's highly customizable.)

Automator (I'm finally getting around to using this... it's very powerful once you figure it out)

Album Art Widget (Dashboard widget that looks up album art for your current song and adds it to your song file tags)