Socrative is a wonderful, free online tool for student engagement. Socrative activities can be used for students to work independently, in pairs, or groups. Sign up for a Teacher account and student login using a "class code" you provide to them. No student sign up required!! It works on smartphones, tablets, and pc's. There are free Socrative apps for the program to work on smartphones.
Questions in Socrative can also be used for group competition using Socrative's famous "Space Race". Watch your class come alive as student work cooperatively answer questions and move their rocket across the screen. The group with the most correct answer will propel their rocket the furthest.