Algodoo

Tap play, and count the collisions, the number of collisions is equal to a certan number of digits of pi, for example, here, the grey block is 10^6 kg, and the blue block is 1 kg, by counting the collisions you can get some amount of digits of pi. Here are some instructions if you wanna make it yourself:
1-Make two blocks, and a wall, and make everything (exept the floor) have a restitution of 1, and set everything (including the floor) to be frictionless
2- Set one of the block's mass to be a power of 100, not a power of 10, 100 (ex here it's 100^3, so a 1 followed by 6 zeros) and the other one's mass to be 1
3- Set the heavy block's velocity to be pointing at the wall and moving.
4- Play around and count the collisions.