Jelly Quest

A game much like Q-Bert, in that you control a blobby Jelly wotsit who has to leap around tiles, turning them from red into green, the trick is that there are loads of different types of tiles: some turn all the surrounding tiles green, some swap between the two colors every time you leap on them while others remain green; some explode if you stay on them too long; others automatically make you jump in a certain direction.

The puzzles are ingeniously worked out and incredibly addictive. A short version was released on PD, while the full version was released as shareware.