WP Hashcash
Not a trackback spam solution, of course, but WP Hashcash is a cute defense against comment spam by requiring a proof of work from the client.
Of course, as soon as comment spammers bake a JavaScript engine into their spambots, it’s all over, so Hashcash isn’t really breaking out of the “arms race” model of spam prevention. (But it does represent an impressively large leap in that race, so it’s likely to be quite effective for a while.)
The one is cuter. It uses ajax and self decrypting javascript and a blacklist:
http://elliottback.com/wp/archives/2005/10/23/wordpress-hashcash-30-beta/