There were around 31 hacks which developers came up with and the Brainiest Hack was the one allowing collaborative annotation of large sets of images without having downloading whole set of photos by any user.
One of my favourite parts was seeing people collaborating and seeking skills in other hackers in order to build a strong team. It was much less of a competition than working together to show what’s possible in 24 hours if you let developers loose without any product or management overhead and boundaries.
Also there was a hack Desktop Wallpaper Love which creates wallpaper from images pulled from Flickr (ok its fine till here) and filtered through Yahoo! Buzz feeds (this is interesting).