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 only used in the construction of Sticky Pistons. 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
Slimeballs are used to craft sticky pistons. A demonstration video showing this.

Trivia

 * Before Beta 1.7, Slimeballs had absolutely no use.