Having only had the site up for a few days, I was VERY disappointed to see that I already had my first spam regsitration (in laymans terms, spam bots who register accounts on forums all over the net so they can post web links back to their own spammy sites, usually for porn, pills or real estate, though I did see one recently for Sheffield Electricans!).
Anyway I decided to do something about it, and having done a little research I came across a great little joomla plugin called OSOLCaptcha. It combines the standard Captcha image method (squiggly writing which bots find it hard to read) with something I had never heard of before, BotScout.
This is a very simple tool which just checks the username / email address / ip of people registering on forums etc. It monitors for patterns of bots registering loads from one address, or using the same details on many sites, and when you run a check, it will return a score. Based on that you can simply bomb the registration out.
I would recommend this is used by as many people as possible because quite simply, the more people use it, the easier it will catch the spammers and be more accurate in recognising them.