Dropbox is a sync, backup, and file sharing tool. It is available for Windows, OS X, and Linux, and will seamlessly sync files among all three platforms. Anything you put into your Dropbox folder gets synced to your online storage as well as any other computer on which you have installed Dropbox. Whenever you change a file in your Dropbox folder, Dropbox syncs the changes. It also saves copies of the old version on the server, so you can use Dropbox to rescue old versions of files, or even files you deleted.
But here is the best part. I am working with several co-counsel at the moment. Instead of emailing them a copy of every document, with Dropbox, I can share a client folder, which then appears in their Dropbox so that we share a single copy of The File. If they don’t want to install Dropbox, they can still have full access from the web interface.