Here is a list of my must have jailbroken iPhone app's for v3.
Installous by Appulous allows you to download any "real" iTunes Appstore app for free, and they are legit, they have thousands.
GV Mobile (which recently got yanked from the appstore) is only good for JB'd phones.
Categories lets you make folders so you dont have apps all over.
FiveRowDock lets you have 5 icons in the dock.
Five-Column SB lets you have 5x5 grid of apps.
MIM lets you replace AT&T with whatever you want.
APTBackup lets you back up your Cydia apps.
MPlayer lets you play unconverted .wmv .avi etc. files
OpenSSH lets you have file access to your iPhone to edit/add/remove files.
CyDelete makes it easy to delete Cydia apps.
SBSettings - Allows easy access to turn on and off 3g, Edge, Wifi, SSH, Reload Springboard, shut down, etc.
iMobileCinema - Allows you to play some (not all) flash within Safari
Stack - Allows your last docked app to be a stack of apps that pops out so you can have multiple things in it.
MxTube - Allows you to download Youtube videos for later playback.
Reminder - Allows you to see if you have email, voicemail, missed calls, ect. on your locked screen.
xGPS - Free turn by turn spoken GPS
dTunes - allows you to download and play music/torrents through your iPhone
Backgrounder - allows you to run applications in the background, for example, if you like to listen to Pandora radio you know that you cant have anything else open while listening. With this application you can open Pandora, then minimize it while it is still running in the background and then open another application while still listening to Pandora.
Winterboard - allows full customization of the GUI and allows you to download pre-made themes to your iPhone from Cydia (I currently use Glasklart).
Emulators - I have the psx4iphone and they have NES and Gameboy Advance, I am still tweakin mine to get Grand Turismo 2 to work properly, but it loads, just lagging right now.