The 2009 SXSW PepsiCo Zeitgeist Twitter Visualizer (long name isn't it?) was a great experiment during the 5-day South By Southwest symposium/conference/party in Austin, TX. Developed by Slash7, the site contains a number of dufferent visualizations using the Twitter APIs.
These real-time visualizations were able to capture only Tweets about the event using the hash-tag #SXSW and highlight the popular topics and show where people were Twittering on a map. PepsiCo has a bigger site with some additional content at http://pepsico.com/sxsw
Found on Information Aesthetics and of course, Twitter.