Akismet is pretty good and effective, but you have to
watch out when it thinks you're a spammer though. Suddenly there's a lot of WP blogs you won't be commenting on.
I think my domain had been previously owned by someone who was A Bad Person, because I started out blocked, it seems. However, once I figured out what the problem was, I got pretty
quick and effective response from both Matt Mullenweg and WordPress.com support, so I can't really complain. That they have got a useful and responsive support tail makes a pretty big difference to me. I'd been a big fan of Akismet, then pretty bummed out by it, and had my faith restored in just a few days.