Adobe Education Exchange: access many resources for teachers including lesson plans, educational videos, and more. Free to use, subscription required.
Class Dojo: marketed as "Behavior Management Software," Class Dojo allows you to use a reward/penalty system in real time in your classroom. Students can watch as points are assigned to their avatars based on such factors as leadership, class participation, helping hands, etc. Free to use.
Google for Educators: a great collection of services and tools designed specifically for educators. Free to use.
Study Hall App: create an online "study hall" session. The teacher can upload presentation slides before class begins, post questions on an interactive whiteboard, post comments on uploaded material, and more. Free to use.
SuperKids: a collection of educational tools. Focused on grades K-4. Free to use.
Teaching Channel: a great collection of videos made specifically to help teachers develop new lesson ideas, improve teaching concepts, and reinforce knowledge of key topics. The videos are categorized by grade and content area. Free to use.
Testmoz: an online test generator. Allows for multiple choice, true/false, multiple response, and fill-in-the-blank questions. Students can log in on a custom url to take the test, and scores are reported directly to you. Free to use.