I recently found out about a new website that is constantly having design challenges. Find out more at a jump:
This website is called Springleap. Every month they are holding Open Theme Tshirt Design contests and a few more challenges on a particular topic with some great $$$ prizes.
My entry to the Open Theme Design Contest of April is this cartoon map of London, that you can see following this link, and scrolling down to the April Open These Design Contest.
You can easily vote for any design you like logging in with your Facebook account, or create your own account to participate in other contests:
To vote just select the desing you like and give it a score from 1 to 5: