Chat application that will feature HTTP requests made from the front end to the back end, as well as web sockets through Socket.IO, which will allow for automatic push notifications from the server to the client.
I can post a new message and it will automatically show up in my client, but in the second client as well, and vice versa. I can open up as many clients as we like, and it will scale. I use MongoDB and mLab to make the data persist.
Most of the node concepts touched on by developing this app.