Slimeball

The Slimeball is an item dropped by Slimes. Its only use is to make sticky pistons.

0-2 slimeballs drop when a Slime is killed. As slimeballs come from a mob, they are a renewable resource.

Player-invented uses
Some multiplayer servers use slimeballs as tokens or cash, since they are otherwise useless and more immersive than a number reported in server messages. The ability for players to get free slimeballs from slimes may make this undesirable.

When in single-player, some players use them to show how many slimes they have killed.

In some adventure maps, slimeballs are used to keep score.

As a Crafting Ingredient
Jeb posted on the Minecraft Forums that slimeballs will be used to craft sticky pistons, and he later released a demonstration video showing this.