If you run a Drupal site with any kind of user submitted content, you have undoubtedly encountered spam. Bots love to spam your comments and user registration forms. It's a problem we deal with on all our Richir Outreach sites, and solving it isn't as easy as you might want. Fortunately, there are a couple of great modules which, when used together, can cut the spam down to a minimum.
The first is the Mollom Module. Mollom is available for many different content management systems, and uses intelligence to determine if something is spam or not. Mollom by itself can do a great deal at determining what is spam, and what isn't.
Another great module is Honeypot. Honeypot has a hidden field on it, which users can't see but bots can. The bots fill it out, and then get banned as being bots. Honeypot is a great, lightweight add-on which catches some of the smarter bots that Mollom misses.
Together these modules can catch almost all the spam that hits your system.