First time I saw Klotski under Windows about year 2000. After several years I rediscovered it in Ubuntu. I solved almost all levels, but to be honest in some cases I had to use solver (see Download section). Many interesting levels are in Bricks game too (this game has slightly different rules and defines many types of blocks) and on web sites with sliding block puzzles such as https://johnrausch.com/SlidingBlockPuzzles/.
Subpages listed on the sidebar contain the shortest solutions of these classic puzzles and some other boards.
All screenshots come from Gnome clone of Klotski game.