Dropbox is a free online facility that allows you to share files across a number of computers. This is useful where a number of people need to be able to access the files. It also creates a form of backup which when combined with something such as Apple’s Time Machine is quite robust.
The free facility is quite useful although you are able to upgrade if you run out of space.
I’ve just had a great example of why I think Dropbox is so great. I needed the logo that sits beside this text but it was on my laptop rather than the iMac that I am working on so, I dropped the file into my Dropbox and hey presto the file is available to this Mac and all the other computers that I have Dropbox synced to.