Akismet is a new web service that stops comment and trackback spam. (Or at least tries really hard to.) The service is usable immediately as a WordPress plugin and the API could also be adapted for other systems.
I must say, this has been one of the more rewarding things I’ve worked on lately — when people tell you they’re able to spend more time with their family because they’re not spending 30 minutes a day dealing with spam it really puts things in perspective. If nothing else, I hope this makes blogging more joyful for at least one person.
Anyway, try it out, install it for a friend, link it on your blog. The more you use it the more effective it becomes. It’s a virtuous cycle that will hopefully curb the spam arms race.