I love Edmodo for my classroom management. I can embed video and Google Docs forms as well as upload assignments, etc. When I was home sick one day, I had given my kids directions via Edmodo, and, after seeing my first period's responses, I realized I needed to clarify my directions. ...was able to do so from home and the rest of my classes did everything correctly. Cool being able to teach from home, even when sick.
I, also, enjoy Wallwisher, Wordle, Tweentribune, and Shelfari, in addition to a number of those already mentioned, here.