d20 Combat Advantage Critical Table-

For the D&Ders:

When a PC generates a Critical Hit (by whatever means one does in your game), (have the Player) roll on the following table:

20-11 Double Maximum Damage, Gain a Free Strike, and a Free Move
10-01 +1d4 Damage, and Gain a Free Move

If the DM desires, perhaps modify the d20 roll by Fighting Man Level and/or Hit Dice.