My first week in New York I attended the first ever Comedy Hack Day, joined a team, and helped build a web app to keep track of your politicians fundraising schedules. Pop in your zipcode, find your lawmakers, and ask they why you weren't invited to the party. Pics or it didn't happen.
This little Rails app takes all the subway entrances in NYC (1904 in all) and plots them on a Google map. The map is linked up to full text search, so you can search for a street or a line to find all the entrances near you.
As an excercise in my web development class, we created this Rails app called datesr, a dating site for hipsters. You can sign-up, log-in, connect with potential "mehs," and send in-app messages.