Slime Block

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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.

Code

//called at the end of Entity.moveEntity()
public void onLanded(Entity e)
{
    if (e.isSneaking())
        e.motionY = 0.0;

    else if (e.motionY < 0.0D)
        e.motionY = -e.motionY;
}

//called at the end of Entity.moveEntity(), after the previous method
public void onEntityCollidedWithBlock(Entity e)
{
    if (Math.abs(e.motionY) < 0.1D && !e.isSneaking())
    {
        double mult = 0.4 + Math.abs(e.motionY)*0.2;
        e.motionX *= mult;
        e.motionZ *= mult;
    }
}