Advance Task Killer - stops unused processes and therefore lengthens battery life and frees up resources on the phone for applications you want to run.

Handcent - replaces default text message app with one that is much more customizable. You can set fonts, colors, notification sounds, etc. for each contact, and it includes some pretty good widgets. It's just like Chomp, but I prefer this one for some reason.

Astro File Manager - also has process killer, image viewer, and batch uninstaller

Last Call Widget - Displays the last phone call made or recieved with one-touch call history or two-touch redial.

Brightness Level Widget - By CurveFish, who also develops tons of one-touch settings widgets for bluetooth, auto-rotate, silent mode, etc.

WIFI Buddy - User friendly WiFi network app

Google Goggles - Image based search, barcode reader, and probably does a lot of other stuff, too.

Dropbox - Backup & Sync with your PC, 2GB free online cloud storage.

Zeam Launcher - replaces the desktops and the Dock with one that is more customizable.

Zedge - Free wallpaper, ringtone archive

...and my personal favorite:
The Moron Test