The one I'm making right now [1]! (This is probably the answer in everyone's minds.)
[1] Crate for file-sharing - http://letscrate.com/
[1] Crate for file-sharing - http://letscrate.com/
The main reason this thought came up is being out of work (You don't hire a Ruby guy and then say "Hey, make this PHP script, Magento, go fast on two servers so it can handle 1000 concurrent requests") I have nothing really exciting to do. Figured I could kill days while looking for a gig with screen fulls of pretty colored code.