The more time I spend on it, the more I am beginning to think that I saved the best for last.  gooru calls itself, "A Free Search Engine for Learning," but oh it is so much more.  Sure, you can search for resources based on a key-word search like other search engines, but the heart and soul of this site is its "collections." There are several collections created that are free for anyone to use.  In these collections you will find videos, documents, diagrams, and interactive activities for such subject areas as math, science, and social studies.  As if that wasn't enough, if you create your own account, you can make your own collections that have the resources that you may have already made or collected over the years.  This could be an awesome resource for you if you are thinking about flipping your classroom.