While there are numerous anti-spam plugins out there, WP-SpamSheild Anti-Spam is a completely backend solution that promises no CAPTCHAs. It’s easy to setup and blocks comment spam before it’s ever posted. This includes ping back and trackback spam, too.
What I liked here is that it doesn’t just work on blog comments. It works on your contact form, too. Nothing like giving spam the ol’ one, two punch! And it promises to work like a firewall against bot-generated comment spam.