Check out - iCall -
iCall - Free iPhone VoIP Watch the Video. Steve says, VIOP Apps will be allowed on AppStore. iCall is working towards that.
Right now you can download to your computer.. Windows 2000 - XP and Vista. Mac coming soon.
Its Free. make calls from your computer anywhere US and Canada.
Make your first call. You will get invite for iCall Pro free. You will get a new phone number.....you will recieve calls on your computer...you can forward calls to your iPhone number..
Want to make International Calls... Go to iCall web site and go to your new account. Enter Credit card details and add some money to your account. Cheap like 1 cent a min to UK. Check international rates page.
I know you can talk on MSN messenger and Google Talk. But this calls from your computer to a Phone not their computer.
Whats great about the iPhone iCall App is that when you recieve a regular phone call you can switch to WiFi on the call on the fly.
Check it out. Chris