Gambler

You can use any combination of dice to make a dice roll as long as the maximum that you can roll equals the maximum you can roll as listed. For instance instead of rolling 8d6 for a Fireball you can roll 12d4, 6d8, 4d12, or even 4d10 + 1d8 etc. 

If you can use this feat on attack rolls, saving throws, and ability checks