Ignoring previous "iPhone VoIP" apps like Jah Jah, which are only VoIP after you connect to their analog to VoIP bridge, this Fring app seems to be the first real VoIP solution for the iPhone/iPod Touch. To get in on this action you need a jailbroken iPhone, Wi-FI access, and you'll be able to call regular phones using either SkypeOut or SIP (your prices will be determined on which SIP provider you use and how much SkypeOut charges for your particular call). Best of all Fring has built-in IM capabilities for Skype, Google, ICQ, MSN, Twitter, AIM, and Yahoo contacts. The download and install are free. [Fring Install Instructions]
With all it's faults... and there are a few. I still say the best way to go is Skype. It's still free Skype to Skype and I pay 30 something dollars a year to call any landline or cellphone in Canada or the US. There are no fees after that. I also top up with about 14 dollars every now and again to make quick calls to South Africa... those calls are usually just to get my family to turn Skype on. I think that over all it's still the best of the VOIP market.