The Pirate Bay was affected.
So was Twitter.
Other sites that went down were Reddit, Gawker media sites, StumbleUpon, Yelp, FourSquare, LinkedIn, and Meetup.
Strangely, Fark was down, but it was a planned outage to move equipment. It just coincided with the leap second crash.
Most of the websites that were affected are back up, or will be soon.
(Image credit: Flickr user Alan Cleaver)