There is the whole piracy thing, of course, but in my case I jailbreak the phone to get some functionality I otherwise couldn't.|
The one program I really can't live without is AskToCall, which puts in an "are you sure you want to call" dialog after tapping on a name in contacts. I found I was making way too many random calls without it.
The rest are just icing on the cake for me. iFile (file manager), iTransmission (bittorrent client, only about half usable, but will do in a pinch), SBSettingsToggles (toggles for various functionality from the main screen).