So it's called DropBox and its software creates a folder on your machine that's linked to their server where you get 2GB of space and the option to buy more. Anything you put in the folder automatically uploads. The handy bit is if you install the same software on another computer the files automatically fill its own folder so you can use it to sync files on many machines without any effort.
You can also let other people download files from the server which is what I'm using it for to distribute zip files to folk. Pretty much like those other websites where you can host huge files for download by others really.
So the planet bit: if you click this link you get a free 2GB account and we both get an extra 250MB. You could even delete your account if it's not for you, I'll probably still get to keep my bonus.
I've also heard that Sugarsync is a good one. Any other similar and better services out there?