A VoIP company called iCall has developed a program for the iPhone that enables seamless switching of calls from GSM to WiFi. Meaning: iCall will prompt you to switch your phone calls, mid-call, to WiFi so you don't waste your minutes. Meaning: It's Free! Though the desktop version of iCall is only available for Windows (Mac Support is on the way) this will probably be one of the more useful VoIP clients for the iPhone.
People who are familiar with T-Mobile's use of UMA and HotSpot@Home technology will tell you that calls over Wi-Fi are not only free (with the right plan), but often times even better. Using VoIP instead of your minutes would help ease the drain for plenty of users who are always near WiFi and potentially offer international travelers free calls back home.
Currently, iCall is in private beta but plans for a public release very soon. In fact, you won't even have to jailbreak for this app because the folks over at iCall are confident it'll be a part of the App Store.