Algodoo

Press W to extend pistons
Stone: Movable
Slime: Movable
Obsidian: Unmovable
Bedrock: Unmovable, does not stick to slime blocks
Piston extends but does not retract blocks
Sticky piston extends and retracts blocks
Works slightly differently than real minecraft, but you can still make some cool things with it
Pistons break sometimes
V1.1: made pistons stronger