I setup a phpbb version 3 forum with a required registration question using an add-on called "Anti-Bot Question". The answer to the forum question would be obvious to anyone interested in the topic area, but would require a bit of research for a spammer. For example, a forum for weather geeks might ask: "What 17th century device measures atmospheric pressure?"

This add-on is a nice compromise between having to approve registrations (which requires your time and stifles spontaneous new users) and open forums.