My project, along with I think 3 others, touched on map visualization. In the past, I've used Google Maps and Google Earth to do geographical visualization. However, I recently "discovered" SVG and ended up writing a visualizer from scratch. Basically, I started with an SVG map from wikipedia and added hover, and coloring capabilities to it. As I was writing this code, I kept wishing I had jQuery like support ... which led me to discover Raphael.js and jquerysvg :)
Going into the event, I was a bit nervous. However, by hour 2, I was pretty comfortable. People were helping each other and sharing their code. I'm not a big Github user. When I ran into a problem, it was kinda cool that I was able to go over to Scott Chacon (he helped figure out the problem in under a minute).
One of my criteria for a successful hackathon is that I can show a respectable demo. In this regard, I could have done better. As others have noted, we did have access to the API for a week. However, since I was busy at work on a project (which, incidently, we are demoing in Vegas tomorrow), I definitely had less than a week. I was impressed with pretty much every demo. What is noteworthy is that most people presented their projects. At the average hackathon I go to, it is probably 50%. It was also nice not seeing start-up pitches being disguised as hackathon presentations. My project could have been better - but whatever - I stood up and presented and that matters to me. I think some participants might have used my code, which made me really happy :)