The iPhone app Fring--which acts as a VoIP client for Skype and other Internet phone networks, as well as a voice and chat interface into IM networks like AIM and Yahoo--is now available for free to everyone.
On the iPhone, Fring can read both your IM and Skype buddy lists, as well as the contacts in your phone's address book. And when you select a contact, you get the option of calling him or her by SIP call, Skype, or standard cellular. VoIP calls obviously don't count against your cellular minutes, but you do need to have a Wi-Fi connection to make the calls.
The biggest challenge for Fring users is that when the app is not actively running in the foreground on your phone, it can't notify you of incoming calls or chats or indicate your presence to buddies. Apple has yet to provide background notification capability to iPhone developers, although we keep hearing it will be in an upcoming release.
Although all iPhone communication apps remain hobbled until background processing is available, Fring does appear to be the most capable voice and text chat app there is for the device, and it's well worth installing.
Fring is also available for several other mobile platforms.