Available for Windows, Mac and Linux machines, Dropbox backs up selected folders from your machine to a remote server. It can also synchronize those folders across a variety of other computers. It is a collaborator's dream.
When you install Dropbox, a folder is created in your Home directory; you have the option of locating it elsewhere on your computer, and it can be moved later if you choose. Dropbox also adds a Dropbox icon in the menu bar (or in the System Tray in Windows), offering easy access to the Dropbox folder, the Web interface, and several Dropbox commands. The Dropbox folder operates like any other folder, except when you put a file in your Dropbox folder, it is automatically synchronized to the Dropbox server. Furthermore, Dropbox doesn't simply synchronize your files, it versions them! Even more, Dropbox keeps a backup of deleted documents.
The coolest thing, though, is that you can share a folder with coworkers, family, friends - anyone. Simply select any folder in your Dropbox and invite the Dropbox users with whom you want to share it. When these users accept your invitation, they will now see your shared folder in their own Dropbox. Like any other folder in their Dropbox, they have full rights to view, modify, delete, or add any files to this shared folder and you immediately see any change they make.Very cool stuff.