Flexbook is a free tool that allows you to "create, share, find and print custom textbooks online." Though it seems to be geared toward Math and Science teachers (or at least feels that way), the possibilities are endless. 

Though some have explained Edmodo as "Twitter for teachers," Edmodo describes itself as being "a social platform for teachers and students to share ideas, files, events and assignments."  Here you can listen to an Edmodo tutorial.  Here you can listen to students reacting to using Edmodo in class.