Some of my favorite apps:
3G Unrestrictor 5 – fool programs into thinking they are on WiFi, allow larger downloads.
Barrel – icon animation transitions.
BytaFont – use custom fonts.
CleanStatus – remove or customize carrier, plus change other status bar items.
Cyntact – user photos on Contacts list.
iFile – file system browsing and sharing via WiFi.
KillBackground – kills all in multitask bar.
MyWi – WiFi hotspot.
NoSpot – remove Spotlight page.
OpenSSH – enable SSH / SFTP access (make sure to change the password for root & mobile).
SBSettings (with “Serious SBSettings” for non-retina devices) – quick access to settings and toggles.
SwitcherMod – ghosts recent applications and makes it easier to clean the Task Switcher.
TetherMe – enables native tethering.
WinterBoard – like a Game Genie for iOS (mostly used for themes).