Similar to YouTube but education oriented. I found some interesting videos on here but I have found the equivalent YouTube videos to have better production values.
It's YoutTube for Teachers!!! TeacherTube is a service that allows users, mostly educators to share educational resources (documents, images, audio & video files) for free. The sites has resources for classroom use, professional development, and even have some posted by students.
Dropbox is an easy way to share files and to back them up. You can access your docs on any computer even without the software installed. You can send an invite to share access to specific folders or files to allow collaboration. It works on multiple platforms including iPad/Pod/Phone, PC, MAC, Android, Linux, and Blackberry
Flickr is a website for storing and displaying digital imagery though it is mostly geared to photography. You can create sets of your own imagery or collect other photographer's photos in galleries. Has many other uses as well. Too numerous to list here.