San Francisco-based sipgate, Inc. is offering software for your jailbroken iPhone to make and receive VoIP calls using any SIP-compatible VoIP service. The service appears to be similar to those offered by Fring and PhoneGnome.
The service is in early beta stage, with several caveats mentioned on its FAQ page. You won't be alerted to incoming calls unless the software is running, which drains the battery with some quickness. The company also promises revisions for the upcoming App Store. It's unclear if the setup suffers the same kind of delay and echo problems that other VoIP solutions do.
Plus, it may eat your firstborn. Seriously. It says it right there.
If you're interested in testing out the product, sipgate is also throwing in 111 free minutes if you sign up for their own VoIP service.
The software is free, and requires iPhone firmware 1.1.4.