Where can I find problems to challenge my UI design skills? [on hold]

I'm still in the beginnings of my career as a programmer, and have always thought I was more interested in low-level back-end type stuff. However, having done a few projects lately on my own, I've been forced into doing more front-end work, and have really enjoyed the challenge of having to come up with neat interfaces to messy problems.

Is there anywhere I can find problems that will challenge my ability to create elegant interfaces? I'm thinking things like:

  • Design an interface to allow a user to easily edit [all-of-this-messy-data].
  • Design a web-based interface to do [X] with(out) the use of AJAX.
  • ...If I had a lot of good examples I wouldn't be asking this question!

Essentially, I'm looking for the Project Euler of UI design. If it doesn't exist, what's a good way to come up with problems to challenge myself?