Here is a list of my favorite CG Challenges across the web, I think they can be very useful for learning and pushing yourself further.
"3dRender.com" Lighting and Shading Challenges:
This is a great website for people who are interested in the subject. There are 26 challenges which you can download the scenes and do your best and post the result for critiques on the CGSociety forums. For newer challenges you should visit this topic on the forum:
"Shaders.XYZ" Shading and Look Development:
Great website and community for people who looking for more challenging subject created by Zeno Pelgrims.
You can download the 8 scenes that are available and play around with them, learn how to solve the problem by learning from solution posted by other users.
"SHADERS.XYZ is a forum created in an effort to unite all look-development nerds from the various render engines out there. The focus is on doing monthly material challenges whilst being as transparent as possible about your process. It's all about learning from and sharing with like-minded people."
Their 8th and latest challenge is the "Sliced Bread" and the deadline is March 1st 2019, so hurry up!
"11 Second Club" Animating Challenges:
"The 11 Second Club is a monthly character animation competition. Animators from all over the world participate, animating a character speaking a line of dialogue provided by the club. Throughout the competition, participants can share their progress with each other and critique each other's work. At the end of the month, everyone votes for whose submission they consider the best for that month. The idea is to give animators a chance to practice their skills in a fun, challenging environment."
"Polycount" Hardsurface Modeling Challenges:
You can find great modeling challenges on the Polycount forums, here is the ink to the 1st Week Challenge:
I'll try to keep updating this post with more interesting stuff.