Dropbox is brilliant. If you just install it on one computer then it is a simple but effective way of backing up some files (the free account gives you 2Gb of space, which comes with web access to your files). But, if you install it on multiple computers (all connected to the same account) then you have just set yourself up what is perhaps the easiest file-sharing system in existence.
Any files copied into the Dropbox folder (located in your My Documents folder) will be uploaded online and then automatically transferred to each and every computer (when online). My Dad (who lives in Australia) and I (I live in Phuket, Thailand) use it for sharing files, mainly photos. This really is a simple way to share files.
Check them out: http://db.tt/zFabAjje