jsFiddle

JsFiddle is great for sharing code and seeing how it works as you manipulate it. My students use this often when trying to help each other out on a problem. You can even save your work for modification later. I have had students turn in assignments on jsFiddle and I readily accept it. My only complaint might be lack of privacy options. However, since we work on nothing that involves sensitive data, and I instruct my students to stay away from sensitive data, this is not a real problem.