Dropbox is a service that allows you to sync your files on your computer with their system as a backup. This also allows you to access the files anywhere. You can also sync the files across multiple computers. This means that you have automatic backup of your files and 24/7 access to your files. I have it set up to sync a folder on my home computer and school computer so I don't have to worry about having multiple versions or forgetting a flash drive.
There are also Dropbox apps for iPhone, Android, iPad and there is one coming for Blackberry. There is an independent app for Palm webOS, but you can also access the mobile site from any web-enabled phone. Imagine being able to access all of your files on your smartphone!
You can also share files with others. I teach EMS classes (EMT and Paramedic) and the course coordinator shares files on it with instructors through one folder and students in another folder. It makes things very easy for all of us.
Plans are as follows:
Basic - 2GB - Free
Pro50 - 50GB - $9.99 / month or $99 per year
Pro100 - 100GB - $19.99 / month or $199 per year
Dropbox is a great service for teachers and students. Access to all of your files anywhere, backup of your files, and the ability to share files.
UPDATE: One feature that is very nice is the fact that it works with network drives like we use in my district. Some of the other cloud file storage and sync systems can't work with network drives. I was very happy to see this was able to be done with Dropbox.
"Cloud File Storage, Sync, Backup" (features similar services to Dropbox)
UPDATE: Mozy.com is another online file storage, sync, and backup service that I just learned about an it also offers a 2GB free account. I haven't used it, but it seems to work the same as the others.