Google Freakin' Voice: Casey's put together a wonderful guide for replacing your phone with Google Voice, using jailbreak apps:

Sure, your old first-gen iPhone may not have the horsepower to run iOS4, but you can hack it to kick your shiny, new, legit model's ass in terms of reception: Use it as a Wi-Fi-only landline. Because a jailbroken iPhone can run apps in the background, you can keep a VoIP app and Google Voice up and running at all times. To get a phone number, we're gonna use Sipgate again, as well as the GV Mobile+ and Acrobits Softphone apps. With those two apps, you can turn your iPhone into a one-stop VoIP phone: incoming calls will ring through Acrobits Softphone, which is conveniently running in the background. For outgoing calls, dial through GV Mobile+'s interface, and your GV number will appear as your caller ID.

This list is in no way exhaustive, so post your favorites in the comments. ATTICA!