I'm sure there are tons of threads about this, but
I like these:
SBSettings - Control Settings (i.e. wifi on/off) with swipe of statusbar
Backgrounder - Lets you run apps in background
Are must haves!
Other ones I like:
QTweeter - tweet/facebook updates from any app
CallMe - lets you put someones photo as a icon (that calls them)
dTunes - Stream music (you pick the song)
Five Icon Dock
Five-Column SpringBoard
Flashlight - brighter than the app store ones
GV Mobile -Google voice
Inspell -spell check
MxTube - view and download youtube videos
3G Unrestrictor - Removes wifi restriction for certain things
iFile - file browser
Safari Download Plug-in
Quick Scroll - allows page scrolling