phpBB spam bots
We used to have a huge problem with spam bots on our phpBB forum. They were able to defeat the captcha mechanism easily and either make spam posts to the forums, or more commonly just create users and leave spam links in the URL field. Which I assume they do so the user list page gets indexed by search engines.
However, a couple of months ago we upgraded the forum software and installed the Anti-Spam mod from Lithium Studios. Since then we've not had one spammer get through to either the forums or the user list, which is a welcome relief.