SFCave was the most addictive PalmOS game I ever played. In SFCave, you piloted a spacecraft through a cave with many upthrusting stalagmites and downhanging stalactites. Pressing the sole control button made you rise, and taking your finger off made you fall. Like all great timewasters, the simplicity was the key to the addictiveness.
Now there’s a 3D version of it implemented in Java, which can be played in your browser. You see the cave from the PoV of the ship, with hills and dips looking ahead of you, and use the mouse-button to rise. This first-person version of the game is, if anything, even more addictive. Consider yourself warned.
(Thanks, Yoz!)