Here's a list:
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A few of the are:
Winterboard for themes, SBSettings for quick toggles, Quickgold for quick launching, iRealSMS for landscape text/forwarding, hClipboard or Clippy for copy/paste, Backgrounder for multitasking, SwirlyMMS for MMS, Cycorder from videos, Snapture for a better camera....
There's tons more you can do..