Shisen (Shisen-Sho) is an interesting variation on Mahjong Solitaire. The aim of the puzzle is to remove all the tiles from an 18x8 grid containing 36 different tiles, each duplicated four times.
The rules are simple. Remove matching pairs of tiles where a matched pair has the same face and can be connected by three or less lines. The lines cannot pass over another tile but can extend beyond the border of the grid.
This version offers unlimited puzzles and all are solvable. Hints are available if you get stuck.