Hi Ian,
If it's for comments and you don't already have it enabled, Akismet is a great option and is included by default with WordPress installations. It will require a little bit of setting up but due to the fact that all of the spammers are reported and blocked against every single Akismet user, it's an extremely effective way of fighting it! There are also a number of WordPress plugins out there that can do similar jobs depending on what you need,
http://wordpress.org/plugins/bwp-recaptcha/ being one of them.
I hope this helps!
Bret