Still the best anti-spam plug-in IMHO

There’s a hell of a lot of them, but I still think the is the best anti-spam plug-in you can get, for the following reasons:

1) It’s incredibly effective – it picks up virtually all machine generated spam (which is more than 99% of it).

2) it picks up virtually all trackback spam too.

3) Virtually no false positives – the only people who can get caught out by this plug-in are those who don’t use JavaScript (which means virtually no one these days), and they get a clear warning about this near the comment form anyway.

4) it’s totally transparent to the commenter, and it’s simple and easy to configure too.

http://wordpress.org/extend/plugins/wp-hashcash-extended/