Socrative - A very fun and easy to use Student Response System to use real time with your students - both for formative and summative assessments. Works on Chromebooks, iPads, and any other device.
Keep Awake - An extension to prevent your screen from falling asleep. Particularly useful for eliminating those moments when a teacher's screen is being displayed for reference and goes dark every few minutes.
Pixlr Express - A fast and powerful photo editing and effects tool. Great for collages, text, and more. Students must be 13 to use the service.