I thought the Pepsi Refresh efforts at SXSW Interactive was really cool.
Here are some photos:
and of course, some free internet
At the other end of the conference center they had a really cool display of what was hot at SXSW. It's a zeitgeist that displays what is hot right now, through Tweets, Foursquare check-ins and Flickr photo uploads.
I love this practical and visual display of real-time data to inform your decision on where to go and what to attend.
Also check out the live site that is displaying this: http://pepsicozeitgeist.com/ . But of course most of the geeks have left, so activity has dropped off significantly.
Although this was merely used as a display, it would be great to turn this into a searchable, navigable mobile application that fulfills a functional role for SXSW attendees.