Finarea has released a relatively nice looking VoIP client that supposedly works with 55 or so different VoIP providers. My gut feeling is that these VoIP brands look awfully similar to be actual different companies but the service actually works. It allows you to make VoIP calls over Wi-Fi connections (no 3G VoIP support) and it seems to work for most. There are some people complaining in the reviews but I had a friend try it and call me and he had no problem.
The MobileVoip app is free in App World but you need to buy minutes from one of the VoIP carriers supported by the app. Check out the details here in App World.
Kudos to Shahzoor @BBin.in for pointing this one out!