sipgate is offering VoIP software for the iPhone that will work with any SIP-based VoIP service. The application works via WiFi, and the company is giving 111 minutes worth of free domestic calling, regardless of whether you have an account with a SIP provider. Article [1]
Links:
[1] http://www.businessweek.com/technology/ByteOfTheApple/blog/archives/2008/06/a_new_voip_appl.html