A new service called Blogs.mu sprang-up recently. It’s kind of like “WordPress MU in a box.” With a very WordPress.com-like setup, you register to become your own blog provider. So you can setup a topical blog community.
It’s totally free, though you can pay for access to various features. (Remove the ads and put your own, use your own domain, get more themes, etc.)
It’s a neat service, and it makes it ridiculously simple to get going with your own WordPress MU site. You can have it running in no more time than a single WordPress.com blog.