[dangrossman]

Craigslist does that with phone verification. You have to take a call or SMS with a code to enter on the website before you can post in a high-spam-risk category. That ties accounts to phone numbers which creates a much higher barrier to re-registration than just e-mail addresses or Facebook profiles, and makes the account traceable if authorities need to get involved.

That's part of the reason I created http://www.dialshield.com, a widget/API that lets anyone add phone verification to their website the same way Craigslist does.