VoxOx

VoxOx is a serious Skype rival, matching many features and extending them further. For example, it is offering inbound US telephone numbers to all of its users for free. The service also offers many other VOIP and Skype-like features, including instant messaging, video chatting, SMS text messaging, callback, and faxing. Its "one number follow me" feature will even let users enter numbers to ring successively in case they're not at the main phone number – similar to Vonage's Enhanced Call Forwarding. They can even create a "this number is not in service" message for telemarketers.
The big disadvantage of the service is its "follower" status. Without capturing a big market share and collecting a huge users database, the basic free user-to-user communication feature becomes useless - if you you have no friends on this network, you have nobody to call...