It's not quite the unbridled version that appeared on jailbroken iPhones some time back, but VoIP software Fring is here at last in an official capacity that even ever-picky Apple can stomach...
For the uninitiated (though you have no excuse to be at this stage) Fring in its purest form is akin to mobile Skype with knobs on. It offers free calling to other Fring users, the ability to call standard land line and cellular numbers using SIP or SkypeOut and incorporates a nifty instant messaging client which is compatible with Skype, MSN, Google Talk, ICQ, Twitter, Yahoo! and AIM - basically everything.
So what do we get in the App Store-d version? Sadly not quite the full package. For example, while all call functionality is in there, calls can only be made over WiFi - yep this is a deal to protect the networks. Furthermore, and this is something of a killer for the IM client (and in fact any IM client until Apple's much delayed and deadline missing Push service takes off), it cannot run in the background so you will only be logged on while you are using the app. Check your email, you're disconnected, look up a number: disconnected, etc.
That said, Fring will work on both the iPhone and new iPod touch (when using earphones with an integrated mic) and it is a slick piece of software. Some early bugs have been reported in v1.0 (notably registering email addresses with full stops in them before the @ symbol) but these will no doubt be sorted out quick sharp. Furthermore once the Push service does launch, Fring - and apps like it - will really come into their own.
Perhaps best of all however is Fring is free, so you've nothing to lose by checking it out...
Fring Announcement (with video)