Doran, the Siege Tower

Oracle Text

Each creature assigns combat damage equal to its toughness rather than its power.

Card Rulings

10/1/2007 Doran’s ability means, for example, that a 2/3 creature will assign 3 damage in combat instead of 2.
10/1/2007 Doran’s ability doesn’t actually change creatures’ power; it changes only the value of the combat damage they assign. All other rules and effects that check power or toughness use the real values.
10/1/2007 This effect is mandatory and affects all creatures.