New Mulligan Rule Effective with BFZ Prerelease
Wizards of the Coast has announced a change to the mulligan rule, effective worldwide beginning at Battle for Zendikar Prerelease:
The new mulligan rule in its entirety:
103.4. Each player draws a number of cards equal to his or her starting hand size, which is normally seven. (Some effects can modify a player's starting hand size.) A player who is dissatisfied with his or her initial hand may take a mulligan. First, the starting player declares whether or not he or she will take a mulligan. Then each other player in turn order does the same. Once each player has made a declaration, all players who decided to take mulligans do so at the same time. To take a mulligan, a player shuffles his or her hand back into his or her library, then draws a new hand of one fewer cards than he or she had before. If a player kept his or her hand of cards, those cards become the player's opening hand, and that player may not take any further mulligans. This process is then repeated until no player takes a mulligan. (Note that if a player's hand size reaches zero cards, that player must keep that hand.) Then, beginning with the starting player and proceeding in turn order, any player whose opening hand has fewer cards than his or her starting hand size may scry 1.
Get more details about the new rule here.