A very, very, very highly recommended complement to Akismet. To effectively and fully eliminate WP comment spam with near-zero burden on your readers, you won't need anything else but Akismet and this plugin.
In fact, Akismet should have included something like this - but we have this plugin's author that's done it one better!
This is the perfect solution that will save you a lot of time wading through Akismet's spam queue for false positives...
(Never make ALL your commenters enter a captcha to avoid spam, do it the smart way with this plugin instead.)
PS - If you have a problem with trackback spam too, check out Simple Trackback Validation.