While I was reading on digg this week, in the comments I came a cross a reference to VoIPBuster. This is a voice over IP provider like skype. However, unlike skype, VoIPBuster offers free calling to landlines in North America and Western Europe. These calls are limited to one minute BUT when you give them a 5 Euro deposit, the duration becomes unlimited.
Here's what their client looks like.
I fired it up just using the built-in microphone and speakers on my trusty ThinkPad. It worked first time. I've just used it to call local numbers and the calls are disconnected after 60 seconds but the quality is great.I'm going to get me a USB phone (like the ATCOM AU-100 above) and play with it some more.