1. Socrative-The Socrative system is web based but there are also apps that students and teachers can download to their tablet devices. To start using Socrative, first go to the Socrative website. Create an account and setup your classes. Then download the Teacher app to your tablet devices or run the lesson from a desktop/laptop computer. Student will also need to login to the Socrative website or download the app from their devices as well.
2. InfuseLearning-This is also a web based clicker type system that can be accessed across many devices. It's similar to Socrative in that you need to create an account and then set up your classes. It also has the feature of a shared whiteboard and to quickly share a link with students.
Both sites are very easy setup for a classroom and will keep your students engaged.