The most obvious use of this technology is when the mixer sends a reference mix to the client via email. Because the music has been recorded digitally, it allows us to render an audio file that can be converted to a smaller mp3 file for quick transfer.
Sometimes the mp3 format is inappropriate or unsuitable for critical evaluation. In this case you are faced with the task of sending a file larger than most email accounts will allow. For this purpose I recommend YouSendIt.com. It is basically a remote server that offers the ability to send larger files across the Internet. They use a secure sever and offer a free service of 200 MB transfers. This is sufficient for effectively moving songs to another client or studio.
For more consistent and professional results, many studios offer FTP services. It is a remote server provided by their Internet provider or web host. It allows the studio to create a "page" specific to the client for uploading and downloading large files. Incredibly helpful when a studio in London intends to send a 4 GB Pro Tools session to a Los Angeles studio for mixing.
Another amazing tool is the accessibility to video conferencing. If you are using a Mac computer you are probably familiar with both the external and more recently, the internal iSight cameras. These cameras can be used to very easily have one studio recording a vocal in New York while the producer in Nashville wants to check-in in real time to see how things are coming along.
One of the coolest inventions to be introduced is an RTAS plug-in called SourceConnect. It is essentially a plug-in that is inserted on the Master Buss of the track within Pro Tools. It sends the output of the music to an online streaming broadcast. The client/recipient is sent an email with instructions for logging in to an account. This will allow the artist, producer, manager or A&R representative to hear, say, a mix recall in real time. They can now instruct the mix engineer to bring up the vocal 1db.