I am a big believer in the use of gamification as a tool for learning. For one, learning is much less intimidating when it is presented in the form of a game. There are countless articles explaining specific design principles and best practices—most of which can be explained through fun and engaging exercises such as this one.
Can't Unsee, by Alex Kotliarskyi and featuring designs by Amanda Hum, is a very simple design game that tests your UI knowledge by asking you to choose the right iteration of a series of designs.

What starts out with painfully obvious examples that you will nail without much thought (like the example above featuring Comic Sans), quickly turns into a challenging experience that will have you examining each design from top-to-bottom.
The game takes you through three increasingly difficult rounds (Easy, Medium, Hard) that all feature 18 sets of designs to compare—the latter of which will require a very close attention to detail in order to master.

Because of the increasing difficulty, this game turns out to be more than just a fun challenge to try to rank up coins. There are some really great examples throughout that remind you of just how much detail goes into a well designed screen.
Although the game is extremely fun and packs a bunch of knowledge regarding UI design principles, there were a few examples I didn't quite agree with. For example:

Something such as border radius can purely be an aesthetic design decision based on the brand it was designed for and shouldn't have an affect on usability. Regardless, these examples are few and far between and overall the game is full of best practices we should all be following.
Can't Unsee is an awesome resource for young designers alike to test our UI skills, and something I have re-visited more than a few times to challenge myself!
Check out the game here, and be sure to tweet your scores @rookidesign!