InterVoip offers different services: cheap SMS, cheap PC-phone calls, free PC-PC calls and even very cheap Phone to phone calls.
You can send SMS instantaneously or program them to be sent at a certain time. It's cheaper than using your mobile hone so it is a good idea to have an account and use this service.
You can use it for free an make PC to PC calls, chat and PC to Phone free calls, but you'll have to insert money if you want to enjoy very cheap calls that can be in really high quality because the calls are done by using your phone. The rates offered by InterVoip are very low although it doesn't have any free destination. Compare rates, it is probably the Voip program with the lowest ones.