Socrative. Socrative was a pioneer in the area of quiz-style student response systems that incorporated responses from students' phones, tablets, and computers. Socrative has offered individual and team games from the start. And the aspect of Socrative that I appreciate the most from a classroom management standpoint is that all activities happen under the same classroom ID. So rather than giving students a different game ID for every activity, you just have them use the same classroom ID for every activity and then select the activity to complete.
A few years ago Socrative was acquired by MasteryConnect. This morning I went to my Socrative account and noticed a banner announcing that it was acquired by Showbie. I read the full announcement here. From reading the announcement it seems that nothing is changing for current Socrative users.