Maybe it has been around for a while, but I just tried Google's Lively virtual world this morning. Second Life it is not, nor does the client currently work on the Mac (note on the site says Google Labs is working on other clients), but it runs within the browser on the Windows machine very well and does not seem to take a super-machine to work it. (Maybe it might even work on the MSI Wind or the eeePC!)
Because of my Second Life experience, I had no problem creating and editing my avatar, creating a room, and decorating it. It will be interesting to try this out with some educators (and, if we make a private room) with some of our students, if that is possible at some point.
Here is my room "K-12 Educators". I cannot seem to find it in the search, but the URL, if you are trying Lively out, is http://www.lively.com/dr?rid=-5452546736007296038.