Slime Block

A slime block is a special block which makes entities bounce when they fall on one.

Friction
Slime blocks have a slipperiness of 0.8, placing it between regular blocks (0.6) and ice (0.98).

On top of that,

Slime Bounce

 * Falling on a slime block inverts the player's vertical speed, which causes them to bounce upward.
 * Holding Jump while landing cancels the bounce, and makes the player jump normally without taking fall damage.
 * Holding Sneak while landing cancels the bounce, but the player takes fall damage.