For UI tweaks, I love:
Beautiful Widgets (time, weather, temp, some toggle widgets)
Power Control Widget (vastly superior power control widget to stock, appearance and number of toggles is customizeable)
Launcher Pro (adjust number of screens, and different launcher dock...single best thing I've put on my phone)
WidgetLocker (put widgets on your lock screen)
Apps Organizer (can put folders of apps on your screen, saving alot of real estate and reducing clutter)
Games:
Angry Birds
RoboDefense
General:
^3 (media player)
Dolphin Browser HD (beats the hell out of the stock Moto Browser)
Facebook
Handcent (SMS)
Touchdown ($20, exchange email client)
To-do task manager (best one out there by far...absolutely destroys Astrid)
SystemPanel (good application manager, and has a "kill all applications widget for the rare times it's needed)
DroidLight (uses LED flash as a flashlight)
mNote (notes to self)
Mint (personal finance)
Google Sky Map