Unspam makes it harder for spammers to automatedly send spam to your blog by changing the names of the fields in the comment forms.
Unspam by Squelch Design is the simplest WordPress anti-spam plugin you can find for reducing your comment spam problem. Once installed there's nothing to configure, and nothing changes to your visitors: No captcha or silly games. Once installed the plugin will simply randomize the names of the fields in the comments form on your blog and reject comments that are sent to the standard WordPress field names, or where bots have blindly submitted data to the honeypot fields.
What this means for spammers is that they have to do quite a lot more work to send spam to your website. It may also make sending spam to your website unreliable as changes to your theme may upset their spam submission tools. Or they may have to resort to using humans to send spam to your website (not much I can do about that I'm afraid) which will cost them more money.
Additional (planned) features: