What web application should I build this weekend?

I'm participating in the Rails Rumble for the fourth year in a row this weekend. For those that don't know, it's a competition to build a web application from scratch in 48 hours using Ruby on Rails.

This year I'm competing on a team by myself, but have been using Rails for years and professionally for over a year. My friend and I also won the most complete category in 2008, see: http://r08.railsrumble.com/teams/the-loose-knuckles

My problem is I don't have any good ideas this year. So, what should I build? I'll pick the best, or most popular, idea suggested within the next 24 hours and open-source it after the competition.

Thanks ahead of time for your help.