Such as the voip services. Since Vonage does it, and they are the market share leader, I think just about all the other voip services do it.

I do it myself by buying cheap numbers from voip providers such as, currently, icall.com, and routing the call through my own Asterisk server. Probably not for the casual user but many do it this way.

Call my voip numbers and you are actually ringing 5 different phones at once.

But not, alas, the cell phones because they go to voicemail if off, which screws it up. I tried to get tmobile to turn off the voicemail, and have not yet managed to make it work.