Here are the JB apps I have installed and find integral to everyday use. Apologies that the list is kind of long:
TetherMe - much cheaper ($2) than MyWi; does not have WiFi tethering but does have Bluetooth, which is all I need and considerably easier on the battery
3G Unrestrictor - older and better than My3G, imho
ultrasn0w
Winterboard
SBSettings - with Retina Display theme iNzoSB; also use to hide "background" apps like MakeItMine
Backgrounder - still useful for apps that haven't been updated for iOS4
Installous - curiosity killed the cat
Signal - so THAT is where those bloody towers are
MakeItMine - because I am vain and like to see my name in the status bar
Infinifolders
FolderCloser - auto-close folders after app launch
Five Icon Dock
Shrink & Iconoclasm - 5x4 app icons on homescreen at 90%, or any other %, original size
iFile - view and manage system files
Safari Download Manager - allows you to download any file to your iPhone (as can be said for anywhere on the tubes, I am careful what I download is reputable and, perhaps more importantly, iPhone-compatible)
QuickReply - reply to texts in other apps
SMS2Mail - back up my text messages
Notified - keep track of notifications
Locktopus - so people messing with my iPhone won't accidentally brick it somehow finagling with jailbreak apps
SnapTap - take pictures using the volume buttons (!)
PDF Patch - for 4.0.1
EDIT: Updated with new apps as of 8/28.