Here's an interesting way to test your depth perception.
In this game by Seryozha Kotsun, you use the up and down arrows to control your shooter's z-depth, while using left/right and space to turn and fire.
The goal is to clear out the surrounding "bubbles" within the time limit without getting hit by the sliding bars. Bonus bubbles provide extra energy points and clock time.
The design is simple, clean and clever - and looks great in motion.
Play the game on Kongregate.