Described on the Google blog and already supported by many toolmakers, basically a link suitably marked won't be followed by the Google spider:
<a href="https://www.example.com/" rel="nofollow">discount pharmaceuticals</a> site.
Great stuff. Probably take a while to have an effect on the spammers, and then they're bound to come up with some new unpleasantness. But this certainly shouldn't hurt.
A solution using metadata, who'd have thought it?
PS. Phil has a link round-up.