A Tale In The Desert (not a headset game) (Score 2, Informative)

As others have said, there are tools such as TeakSpeak and Roger Wilco that will deal with the voice chat side of things.

But what game to play? I'd suggest A Tale In The Desert. It's nice because it has a very laid-back feel to it - there's no combat, and the emphasis is on long-term co-operation with your friends to further the development of your avatar (the web site, as well as the excellent fansites and wiki explain all this much better than I can).

It runs on Windows and Linux, and doesn't have to run full-screen, so it's great for leaving running in the background so that you can get some work done while your avatar is busy feeding the camels or waiting for papyrus to grow. The chat system is also pretty good, for those of is who don't want to inflict their irritating nasally voices on the rest of the world. :-)