This probably goes without saying, but the incredibly popular and growing social sharing and media site, Pinterest does nofollow links to third party sites.
The nofollow attribute was introduced in 2005 to fight comment spam and other types of link spam. But it has grown into its own beast.
It might help Pinterest from being abused with link spam, but won't stop spammers and their larger issue of copyright issues and DMCA takedowns.
Forum discussion at WebmasterWorld.