According to the Diagnostic and Statistical Manual of Video Games (DSM-VG), it's a known fact that gems prefer to mingle in linear clusters of three or more, after which they cease to exist and exert a downward gravitation pull on other gems around them. However, they can't move around on their own, which is where you come in. In Drop Swap, a match-3 puzzle game by Aaron Steed, you mingle around a field of gems using the [arrow] keys. As your shadowy character walks into a gem, you swap places with it. Moving three gems in a row removes them just like any match-3 game, but you have to plan your moves carefully, because you get a limited number of steps to make a match!
Clearing a match restores your full stock of moves, but running out of steps means you've failed the level and have to press [R] to restart. And unlike most other Puzzlescript games, there's no undo key, so every move matters. In each of Drop Swap's five levels, your goal is to either clear all of the marked gems (with a plus that unfortunately covers their colorblind markings) or drop a number of grey gems to the bottom row. The fourth level introduces skulls that instantly kill you if you're next to three in a row, but can be removed by dropping them to the bottom like a grey gem. Though there are only a handful of levels to play through, just enough concepts are introduced in those levels to make them strangely replayable. Drop Swap is a great example of a simple idea done very well.