If you've got an iPhone or iPad, get Temple Run. It's free. Gameplay is simple, straightforward and good. It's 3D, looks good and plays smoothly at all times. Time investment in each game you play is short, perhaps a minute or two, so it's perfect for playing while riding the bus or something. Each time you run, you earn coins, and coins are used towards improving powerups and other things, so even a short fail run isn't totally wasted. Powerups are rare but very useful, so it's worth the time to max out the powerups. Each time you earn an achievement, you permanently increase the final score multiplier.