Type 20 is a new format that some friends and I threw together because we were bored with our decks.
You build a deck of 20 cards, no more no less.
Your starting hand consists of 7 cards, and you begin with 20 life.
After you have looked at your hand, you have the option of taking a paris mulligan. You can only mulligan once.
All other rules are the same.
This format is tons of fun because it is so fast. Knowing that you are on a 13 card clock forces you to build interesting decks. Decking some one becomes easy and sometimes broken.
This format has a variant called Booster20. Instead of using the cards you own, each player buys 3 booster packs. You can use any number of basic lands you already own, and you proceed to make a 20 card deck with the cards from the boosters and your land.