Socrative or Exitticket. However I find Twitter a bit more engaging due to its social aspect.
So it all happened during a Physics class. The topic was temperature conversions. Since I knew that most students were good at temperature conversion from past courses I thought of this class as a review. So I planned to use Twitter contest. I sent out a temperature reading and they would convert it to another unit. To avoid them cheating and using Google search to get an automatic answer I had them solve the problem on paper and take a snapshot of the process and tweet it. We kicked off but we encountered a problem. For some reason, two of the teams were having issues with their mobile Twitter (I only allow them to use handhelds for this particular activity since a PC or Mac server might send tweets faster than a mobile and some of my students have handhelds only.). Their tweets were not sending out so the game was a bit uneven. After three trials we realized it was not going to work.