A free program for Android, by AlgoTech.

You are playing against a real person on the other side of the screen. The dice will be rolled by both of you. Each of you can roll as many dice as you wish. The goal is to score as close to 21 without going over. You will both get points for the dice. The player who gets the exact score of 21 wins the round. If you get closer to 21 than the opponent, you win. If you go over 21, you lose. 

You will be able to play with the same opponent again in the future. You can also play with other players on the same device.