From the Akismet website:

We can't stand spam. Who can? You have better things to do with your life than deal with the underbelly of the internet. Automattic Kismet (Akismet for short) is a collaborative effort to make comment and trackback spam a non-issue and restore innocence to blogging, so you never have to worry about spam again.

From the Akismet stats page:

259,612,545 spams caught so far 1,719,297 so far today 92% of all comments are spam

For me, Akismet has caught 664 spam comments since I installed it.

Thank you, Akismet!