Minecraft wiki entry for Slime_Block, associated text: A zombie bouncing on a slime block. Click to view animation.
Image posted by Jeb over Twitter posted to Imgur.
The 14w02a Banner showing slime blocks in a slime shape. Obsidian is used for the eyes and the mouth.[2]
Testing of bounce heights. X axis is the start height in half blocks. Y axis is the return height in half blocks. Red line at the top is maximum possible bounce height in half blocks.
Crafting a Slime Block in Minecraft: Story Mode