BuddyPress.org

I’d say the noteworthy features for Anti-Splog are their API and limiting number of signups per IP per 24 hours.

I don’t really like the “changing signup page location every 24 hours” feature, although I understand the rationale, I’d prefer a permanent location for indexing reasons.

The rest of it is doable.

To make it seem like their plugin is uber-spectacular, they’ve created a flowchart :p

http://wpmu.org/wp-content/uploads/2010/01/Anti-Splog.gif