here's what i fuck with:
chompSMS: texting program that seems to work better for me than handcent or the default program...you can customize the look of everything, change fonts, etc.
SpoofApp: Call anybody and choose the number/name that shows up on their caller id...gives you the option to record the phone calls, and even has a voice changer (which is kinda shitty)...AWESOME for pranks, but it's kinda evil hahaha
PhoneMyPC: lets you control your computer from the phone...REALLY dope...$10 though
ringdroid: make your own ringtones from your mp3's...hella easy and useful
Weather Widget Donate/Beautiful Widgets: put a clock on your home screen that shows up to date weather information wherever you currently are...dope.
pandora radio: free internet radio customized to your taste
Space Physics: fun game
AP Mobile: News Articles with customizable categories
Super SMS Faker: You can fake ingoing and outgoing texts so you can bullshit yourself out of a situation...set it to timers, etc.
Trapster: Check out your local area and see if there are speed traps, dui checks, etc.
advanced task killer: keep background programs from taking up phone memory
nesoid lite: nintendo emulator...hella fun.
barcode scanner: very useful...price check anything with a barcode
LED Flashlight: turns the camera flash on so you can use it like a flashlight
backgrounds: get hella wallpapers for the phone
google sky map: useless, but still tight...point the phone into the night sky and it'll tell you what you're looking at.
places directory: best app i've seen for searching the local area for restaurants, gas stations, etc.