It's called "Drop Box" and it's a sort of dynamic folder that syncs between many users from anywhere in the world! Any file that is saved or altered in my Drop Box folder will be updated on all of the other users' computers as well! Better yet, I can have several folders, each setup for different projects, with an entirely different group of users!
For instance...
Folder: "Charles Billingsley" is setup between myself and my producer friend, Adam Lancaster.
Foder: "CPB" is setup for my clients at Liberty University and their upcoming live album I'm mixing.
So, anything I place in the "Charles Billingsley" folder will be updated on Adam's computer. If I save a mix file in the "CPB" folder, it will notify the other users that a change has been made. I quickly hear back on the approved mixes!
TIP: If you're mixing a project (or producing for that matter) for a client, include a document called "progress" and keep track of how things are coming along. Include questions you have for your client or request mix approval on a song you've mixed. The document becomes a virtual time line as you work on your project.
I must thank my friend Adam Lancaster for introducing me to Drop Box!! Thank buddy!