RECIPRO Sliding Box Puzzle by Masahiro Nakahashi. The object is to organize the numbers in order by swapping the circled number with an adjacent number.
The 2x3 grids go very very fast. The 3x3 and 4x4 grids are obviously more difficult.
The best part of this game is the Battle Mode. This sets you up with a random player to see who can solve it faster. This is an essential part of the game in order to graduate from the 2x3 grid and the 3x3 grid.