James Mishler writes a blog called Adventures in Gaming, and he has come up with a chart that depicts the history and geneology (?) of Dungeons & Dragons. It is well worth a look. (Unfortunately, Blogger won't let me set up the image to click-thru to James' post, but there are plenty of links.)
[Click Through to Adventures in Gaming]
There is a lot of other good material on this blog as well, and I plan to spend some time exploring it.