The Chat Gem
The Chat Gem is an easter egg from Blizzard implemented on the Diablo II Battle.net chat screen, the Warcraft III Map Editor and now the Diablo III main page. It's a gem you can click to activate and deactivate it. Switching from its dormant blue state to it's activated glowing purple state.
The current possible responses include, Gem Activated, Gem Deactivated, and the more rare Perfect Gem Activated, and Moooooooo!.
The Chat Gem has inspired massive amounts of controversy as to what else it might do, some of the more popular theories being activating the Secret Cow Level, and more recently bringing the release date of Diablo III closer.
Whenever asked about The Chat Gem the only response we ever receive is, "It is working as intended."
It will truly carry with it a mystique and abundance of theories for the rest of time.
In an interview at Gamescom 2011 by GameTrailers, Jay Wilson stated that "the chat gem is not coming back" to Diablo III. He also said, "I will definitively say, the chat gem did nothing" in regards to its functionality in Diablo II.