Inspired by a conversation I overheard at work, I have recently been playing with Voice over IP telephony. VoIP is about to explode over here in the UK. By assigning an IP address to a phone, calls can be routed through computers using PBX software such as the excellent Asterisk. Once a call is handled by a computer pretty much anything can be done with it. One good example is VoIP User. They offer outbound calls to landlines around the world allowing its users to experiment with VoIP. It is completely funded by incoming calls made to the service. There are currently some restrictions such as 10 minute call limits to prevent abuse.
The big bonus of VoIP and the SIP/IAX protocols is that calls can be made directly between phones for free. Once it hits a critical mass of users, the current service providers are going to have to find something other than expensive call rates to offer their subscribers.