I like using the Disqus comment system here (and as a commenter myself elsewhere). It’s used across many blogs, so you don’t have to individually register with all of them, and comes with some built-in spam filtering as well. Once it’s installed in WordPress, it overwrites the built-in commenting system, but not perfectly – the comment count still shows the old, now defunct, system.
If everything I just said made your eyes glaze over, feel free to ignore this. If you’re a blogger and this has been driving you nuts, check out this short video I made on how to show Disqus comments correctly in WordPress.