I’ve had my ups and downs with Akismet, but I gotta admit that after catching over 100,000 spams for me, I can’t seem to part with it. Sure, it catches a few that it shouldn’t from time to time. And sometimes I don’t check my spam queue as thoroughly as I perhaps should. But in the little over two years that I’ve been running this site, it’s caught over 100k in spam messages.
I’m fairly certain that there are other spam catchers out there that perform just as well, but Akismet is integrated into WordPress, so it gets the easy nod. I’ve tried programs like spam karma and really didn’t like it at all. Akismet is easy to use and aside from the occasional <1% false positives, it does a really good job.