I just installed Fring for iPhone, and it is the best community messaging application I've seen for the device (what is Fring?). AND, it supports VoIP in the form of Skype and SIP (!). This is the application I've been waiting for, and is a really compelling reason to jailbreak your iPhone. I fired it up and gave Victor A. a call using SkypeOut. Other than the standard latency that one expects from a Skype -> mobile phone connection, everything worked perfectly, and the sound quality was quite good.
Fring even runs in the background, which is really cool. I'm not sure how much battery power the thing sucks down, but it certainly is really great to be able to use Skype on my iPhone! I'm sure the 3G version will give the iPhone enough bandwidth to be able to take advantage of VoIP when out of WiFi range.
Fring supports Skype, SIP, Yahoo! Messenger, AIM/iChat, MSN Messenger, Google Talk, ICQ, and Twitter.
Congratulations, Fring developers! You get a spot on my iPhone's home application page.