This is one of the most useful applications out there for people that want to share files between multiple computers or share larger files with friends.
Basically what it does it creates a special folder on your computer that if you copy files to it the file will automatically be synced with your free storage space online and then pushed out to all the other computers you have logged in to the same account.
There’s also a web interface which allows you to get at your files without having to install the client.
Over all I’m really impressed with how well this application works. check it out for yourself. http://www.getdropbox.com/