Spam Insight
I read a brief but interesting article on the Akismet blog today offering an insight into the current state of web based spam.
While Akismet isn’t a blocking service I personally feel that the large IP ranges identified by the article as solely in use by spammers could do with being blocked.

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I've canned 470,515 spam blog comments so far and rising.
I have a zero tolerance on spam!
Its a fact you'll attract all the things that you lack