Algodoo

Those lego bricks stick together.

How to easily stick and unstick the bricks:
1. Let the scene run (important, the method won't work when not running)
2. Choose "move objects and water" (the tool on the left) - do better not use the drag tool
3. Put a brick on another (or take it away).

How it works:
The rectangles that are on top of the bricks are not straight, but have some angle, so the bricks won't unstick by lifting them up.

It's not perfect (some bricks are unsticking after some time).