My colleagues, Adam Bair, Pradeep Elankumaran, Paul Ostazeski and I ended up building an app within 48 hours for this year's Rails Rumble. We had a couple ideas at first but we decided to try keep it simple to make sure that we had enough time to iron things out. We decided to do a deals aggregator with a little mix of Digg/Reddit thrown in. Smacksale.com allows you to vote up or down deals that you either love or hate.
It's also interesting to note how much planning went into it. The only bit of planning we did was this little mockup that I drew up (I can't draw straight lines for the life of me). Smacksale stayed pretty much true to the initial mockup. If you have time, check out Smacksale (http://smacksale.com).