This is totally off-topic and self-congratulatory. But for the past six months or so I've been teaching myself Ruby and Rails, and just recently shipped my first app.
Glasses is an app that tells you if you'll need sunglasses today. Totally the worlds simplest and most optimistic weather app.
Source: http://github.com/bdcheung/glasses
Demo: https://damp-mesa-5986.herokuapp.com/
It's mobile compatible, thanks to responsive design.
Let me know what you think (or if you have any ideas for new features!)