I love it.
It has been around since 1979 and currently looks like this:
The object of the game is to guess your opponent's character using only yes/no questions.
For example: Il a les cheveux noirs? Elle a les yeux bleus?
It lends itself really well to all languages and is a great way to get students of all ages speaking in the target language.
It comes in various different editions from travel size through to an electronic version, there's an app and even a tea towel...seriously.
Look:
There is also a corn-who-copia of copies available too...if you are Guess Who-ing on a budget....like this...
and this...
Anyway, you can play this in your classroom without spending a huge amount of cash.
If you go to the Hasbro Guess Who website, you can download extra character sheets.
If your printing budget doesn't allow for colour, you can paste an image into a powerpoint presentation.
In the past I've even used photos of teachers, students and celebrities, too.
About 15 years ago I bought cheap versions from the Pound Store and laminated the cards.
I'm still using them.
Students always seem to love playing it.
Enjoy.
PS
I discovered today (from wikipedia) that there is a fan-made version of the game, "Guess Who: The Utley Rules".
In this version "players can only ask about the assumed characteristics of the characters" based on their physical appearance.
This sounds like it might be a lot of fun for the more able students I teach.