Sites that offer creative challenges and contest. You may have to sign up or create an account.

worth1000.com
Photo manipulation contests or challenges. A great site just for creative inspiration.

ibeatyou.com
Offers tons of contests on many topics, photos, videos or text. You can create your own contests here.

illustrationfriday.com
“A topic is posted every Friday and then participants have all week to come up with their own interpretation.” How fun is that?

drawingboard.org
This site is more of an illustrator’s forum with some topics offering challenges (Jams) so participants can compare styles.

If you know of any others, please let me know. I would be happy to include them in this list.