DropBox is a free (2GB) download and enables you to share folders and/or documents with selected colleagues or friends. Rather than having to email often cumbersome files, the holder of the DropBox invites friends via email to a specific folder within the DropBox. Friends can then either add or access these files, but only within the specific folder they have been given access to. Large photo albums, powerpoint presentations or documents are quickly and easily transferred.
If you need more storage to meet your needs, there are premium accounts.
The addition of mobile DropBox apps for iPhone, iPod touch, iPad and Android (Blackberry coming soon) means that you can sync these with your computer. You can also add more than one computer to your account; so you could access DropBox on your home and work computers as well as a mobile device.
The video on the DropBox sites explains it all in two minutes.
I can say that DropBox has been an excellent addition to my toolbox and I have used it at least once a week since March. The DropBox website states that it works on PC, Mac and Linux.