Doctopus!

I just learned of a Google Apps script called Doctopus that makes it easy for teachers to manage and share documents with students or groups. After sharing a document with students, the teacher can keep track of when students last edited the document at a glance. The teacher can also "lock" all the student documents with one click on the due date so that grading can be done, provide feedback, and then "unlock" the documents so that students can revise.

Check out this video tutorial created by Jay Atwood.  Enjoy!