Dropbox is a file storage service that syncs up files between multiple machines. Once installed, you gain access to a virtual folder that will stay synced up and pass along any new additions, deletions, or changes. All the while, the service keeps snapshots of every version of a file that's been changed, which means you can go back and retrieve older iterations.
Dropbox also lets users create shared versions of these folders, so multiple users can contribute or make changes to a collection of group files and make sure everyone is using the most up-to-date versions.
Dropbox has both a Web and desktop component. The desktop software lets you forget worries about re-uploading while you make changes, and feels just like a native folder on Windows, Mac, and Linux PCs. And Dropbox's site lets you get at all your files, no matter where you are.