Ok, so you've probably seen some of these, but I needed a snazzy title. I recently compiled two lists of sites from Richard Byrne's Blog - the site is an amazing resource, and I'm pretty certain that Richard doesn't sleep. These aren't in order of greatness - they all have their own specific applications for education.
Dropbox is a very useful tool to store online files, sync your computers and mobile devices and share files. With all those capabilities it makes it a natural tool for educators!