BuddyPress.org

Andy,

What type of plugins/multiple db’s/caching do you have enable to handle all the users of this site?

Of course WordPress.com which runs MU has scaled quite well, but I know they do some special things on the backend to make that happen.

WordPress (which is what I’ve done up until now) scales VERY well. I’m amazed at how much traffic I can handle on WordPress for $6 a month.