Update (2014-04-04) : The plugin is on now in WordPress's registry and on github, feel free to submit any patches.
There are 100s of different plugins that fight spam for WordPress, however almost all of them require user interaction someway or the other, and usually they require a remote service to verify the user's input.
While I was trying to combat spam on this site, I came across Anti-Spam, great concept, however it still required the user's interaction, and then my gears kicked in after analyzing the 1000s of spam comments that were posted on the site I wrote a very simple plugin to combat comment spam.