Bukkit

Place a piece of wool under a sponge and when someone jumps on it they will get launched, the more wool that is directly under the sponge the higher the player will go.

If you place a piece of wool down one block and to the side the player will get launched away from the off-centered wool block.

If you remove the block under the sponge but still have the blocks going to the side the player will be launched to the side without being launched up.