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).