Columbus Building Age Map
A map displaying the year of construction for (almost) every building in Franklin County, Ohio. The map can be viewed here and the source code for the data collection and vector tile server can be viewed on GitHub.
A "social network" demo with a live feed of messages that are submitted anonymously and vanish after one hour. Under the hood, it's a RESTful web service built with Scala, the Play Framework, and a PostgreSQL database hosted on Heroku. The actual site can be found here and the source code is hosted on GitHub.
"Shitty Yik-Yak 2.0" - Matt Cohen