After visiting several different sites the one I found the most useful is Pinterest. I know it's not listed but I like it because it pools from different sources and depending on who I am "following" it is specific to my interest and class. Another site that had a lot of material was United Streaming.
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I've had a dropbox for several years. I use it to store lesson plans, interactive whiteboard files and any file that I want to be able to access anytime, anyplace. Great for those days when I don't have my laptop or forget my external hard drive.