4 Player Reactor is hands down, some of the most fun you will have on your mobile friends (so long as you have a few friends with a competitive spirit). In short, 4 Player Reactor is a series of short, reaction based games, that you compete in against your friends. The concept sounds simple, but the execution is superb. This truly is one of the most unique experiences I have had on my android device to date, and I highly suggest anyone who is yet to give it a try - to do so - now!
Check out 2 Player Reactor if you want to to try a free version of this app before forking out the big bucks ($2.99)!
In the words of the developer:
Match your reflexes against your friends in many action-packed game modes!