Slimeball

The Slimeball is an item dropped by Slimes. There are currently no uses for it, however they will be used to create sticky pistons in Beta 1.7.

0-2 slimeballs will be dropped when a Slime is killed. As slimeballs come from Slimes, they are a renewable resource.

Player-invented uses
Some multiplayer servers use these as tokens or cash, since they are otherwise useless and more immersive than a number reported in server messages. However, 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 later released a demonstration video showing this.