Flyde from the moment I laid eyes on it. It reminded me of an anciet DOS game I used to play as a kid, where you had to race around in 3D and jump platforms while the level rushes by. [I totally played that game too -- anyone remember the name?! -Ed]
In Flyde there's no jumping per se: you control a white triangle that drives on or flies over a track made out of squares. There are colorful squares and black squares. The black squares are actually "holes" - when you drive over them, you fall into the abyss of space and die.
But it gets a little more complicated than just "not driving over the black squares". Instead of only one track, you have two tracks. When you drive over a square in the color of the other track, you switch tracks! The second track is above you, so when you switch, you're actually flying upside-down.
To keep from driving over the black squares, you have to continuously switch from track to track by touching the colorful squares. You're going pretty fast so navigating between those evil black squares and switching tracks just at the right moment is pretty challenging, and also a lot of fun.
The soundtrack consists of a robot with an attitude problem, but it also tells you how many points you've accumulated. This, along with nice electronic background music, makes Flyde a truly engaging time waster, even if you're not into retro gaming.
Oh, and don't judge me by the score in the screenshot. My top score is actually 12,610 - what's yours?
[Update: The old DOS game is called SkyRoads. Thanks, TnS! Awesome commenting there.]