This has been asked inside a few topics, but has never been given it's own forum topic. In light of the recent announcement, highlighting the increase in comment spam, I thought that this should be brought out.
Comment spam is easily dealt with under WordPress, and DreamHost has kindly posted a Knowledge Base article on this very issue.
This KB article links to a very complete database and tutorial on the various anti-spam techniques available to WordPress users, created by fellow WP user and WP Support Forum admin, Podz.
If you have read the above two sources and still have a few questions about WordPress and comment spam, the WP forum members would love to help you at this WP Support Forum thread.