Proofme is the one I have been using lately. Proofme offers 5 GB of free storage. If you like the service and that's not enough, the prices after that are really affordable. I use it for work collaboration but college students would probably find it useful too. You can collaborate and discuss things that you upload on there with your teams. Some other applications offer the ability to make markups but Proofme allows you to do it in real time. All kinds of things can be stored like documents, images, and videos. You can also integrate with applications like DropBox, GoogleDrive, and OneDrive which makes it all the more useful. It's possible work on projects with as many people as you choose. Anyway there are a lot more features than I'm not explaining. The website is proofme.com