New Yahtzee is a great time wasting app that allows you to play Yahtzee against friends. You can play a game in turns so you don't have to sit down with the time for a full game, or you can play in a blitz to complete a game in a fast paced back and forth.
I love having multiple games going, so I can bounce in and play several turns when I have a few minutes. I typically bounce between New Yahtzee and Domino! when I need to waste time playing a game.