Spambox

What if the major free email providers (Gmail, Yahoo, etc.) set up special mailboxes designated spam-only in an effort to improve their spam filtering? Set up a few accounts, put some links on the web, sign up for a few free services, and wait for the spam to come rolling in. Mail that makes it to the inbox obviously got past the spam filter; simply auto-mark it as spam to account for it according to the already-in-place mechanics.

This may already exist, of course, and Google's spam filtering at least is quite good, but it would be a simple way to get a possibly big improvement. (Unfortunately, spammers could use this same technique to figure out what gets through in an attempt to find holes in anti-spam technology.)