I enter a monthly challenge along with many others at http://www.fujimugs.com
It is a competition for people who use Fuji cameras, though there is now an Open catagory that can be entered by anyone with any brand of camera.
Its a great place to pit your shots against other people and get some constructive feedback of what you could have improved and what you got right!

The title of each shot is the challenge for that month.
Each challenge has a different subject that all the photographers have to try and capture, the winner from the previous challenge has the honour of choosing the next subject.