Many of the VoIP services like Skype and iChat use "softphones," which means the phone is really a program on your computer. You still need a headset of somekind, but the whole set up is a bit clumsy compared to the time-tested "telephone" interface we've been using for, oh, a hundred years or so.
Engadget has an article on an inexpensive (about $45) VoIP phone that actually looks and acts like a phone. The neat thing is that you don't plug it in the wall, you plug it into a USB port on your computer.
The only problem I have with this is that it ties the phone to your computer. A more practical solution is the adapter box that some outfits like Vonage sell. It plugs into your Ethernet jack (your whole house is wired for Ethernet, isn't it?), and any standard phone plugs into the box. So you can have the phone anywhere you have Ethernet access...even WiFi access.