Clickers & Socrative
Engage and assess your students all at the same time!
eInstruction Clickers
Classroom Response System
Socrative
Online Classroom Response System
Student Response Systems
Ready Made Quizzes
Smart Exchange - requires a login, but it's free to register. Can use the question sets in the verbal engage mode.
Quia - Amazing!!! - Free quizzes for every topic and grade level!
Internet 4 schools - Activities organized by Grade/Subject/Skill - Awesome!!!