Software Based VoIP Service Provider

Software Based

ICall.com (iCall - Make and receive free phones calls on your computer, iPhone, iPod Touch, and Web Browser)
-US and CANADA
-Uses a Skype-like soft phone.
-The software is slighly glitchy so some patience is required.
-As of late calls are supposedly limted to 5 min per call (but nothing stopping you from calling again once the first call is up for another 5 min)

MediaRingTalk (Welcome to MediaRing Talk)
-Promotion with free calling to 8 COUNTRIES
-Skype-like softphone.
-Personally I couldn't make any calls with them, when I tried.

TuiTalk (Free International PC to Phone Calls TUITALK)
-Register
-Dowload SoftPhone
-Enter Number>>Watch Ad>>Make Call
-Call limits based on ads served to you

AdPhone (ADPHONE)
-MANY COUNTRIES
-Download SoftPhone
-Complete Registration
-Get 2 minutes free per call , up to a total of 30 minutes per day. You cannot call the same number for free more than once every 10 minutes.