if we are going to get into some Jailbreaking applications i can tell you the must haves.

1. Backgrounder(allows you to run any applications in the background like a Blackberry, for instance listening to music from Pandora(great app as well) while texting.
2. 3g unrestrictor this one is a MUST, you can download this app and bring your minutes down to the lowest you possibly can, because you will recieve and send all of your calls through your internet, which means you pay just a tad over 30 dollars a month(which is the internet price)
3.SBSettings is another good one, it allows you to add or subtract pretty much anything on your iPhone from your computer. its a nice one but may not be for everyone