Make Free Voice-Over-IP Calls on Your iPhone with iCall
You'll also have the option to transfer inbound calls to Wi-Fi, thus saving those pricey AT&T minutes. iCall offers custom voicemail options and access to your existing address book.
You can't get the applet yet, but TechCrunch suspects it'll be available next week as part of Apple's new iPhone App Store. Could be a winner -- we'll have to wait and see. In the meantime, check out the demo video.