iSketch is a simulator that allows two or more players to play in which one player is given a word and they have to draw something that represents that word, while the other players have to correctly guess what the word is. It is very similar to Pictionary.
Doesn't require to sign up; you can choose a nickname and it will remember it.
Has gameplay instructions.
You can create your own room and limit the number of people who can access it.
Games are always finished in one sitting with a time limit, though players can continue on with their game if someone joins or leaves.