RF.com lets you connect your existing SIP, IP PBX and/or IP-based phone systems to your iPhone and make free IP-based calls (Skype, Google Talk, Yahoo!, MSN) from your iPhone. RF.com is a mobile IP voice communication platform bridging the cellular and Internet offerings of the iPhone to provide simple and convenient access to hundreds of millions of Internet communications users over the ubiquitous existing cellular infrastructure.

You must be using an iPhone to see the iPhone version of this app. Try it here