The game that's simple in every way except the puzzles...
Split is a 2D puzzle game in which the player must draw a single loop through a grid of dice-like dots, and split the dots into groups of equal value.
All puzzles use some variation on this simple rule, for example allowing the line to cross itself or two lines drawing simultaneously on separate grids.
The game currently contains around 140 puzzles, but more are continually being created. The final game will hopefully contain around 200, ranging from the easy "Tutorial" to the really hard "Annihilation".