Sorcery, XXR (1)
Common
Meteor Shower deals X plus 1 damage divided as you choose among any number of target creatures and/or players.
"Eenie, meenie, minie, moe . . . oh, why not all of them?"
—Jaya Ballard, Task Mage
Illus. Rick Emond

10/4/2004: You can't choose to do fractional damage to a target.
10/1/2008: A tip on how this math works:
To deal a total of 1 damage, X is 0. The cost is {0} + {0} + {R} = {R}.
To deal a total of 2 damage, X is 1. The cost is {1} + {1} + {R} = {2}{R}.
To deal a total of 3 damage, X is 2. The cost is {2} + {2} + {R} = {4}{R}.
And so on.
10/1/2008: The number of targets must be at least 1 and at most X + 1. You divide the damage as you cast Meteor Shower, not as it resolves. Each target must be assigned at least 1 damage.
Legal in Vintage (Type 1)
Legal in Legacy (Type 1.5)
Legal in Classic (MTGO)
Legal in Commander