Here are many blender contests, but most of them are CG contests. I am introducing some 3d print contests. I hope you will enjoy them.
Engineering oriented, have many challenges
https://grabcad.com/challenges
3D printing service website. I also apply for the endangered species contest.
http://www.myminifactory.com/object/view/517/Category-1
Here is the rendering picture. A little Hawksbill Sea Turtle. The real object hasn’t been printed. I will post it when it’s ok.