Dropbox ( http://www.dropbox.com/
Btw, Hamachi is a great solution too, as far as a VPN, but for this particular need, something more "targeted" seems more appropriate, and just easier. Having a secured, authenticated connection (be it in the form of Hamachi or other VPN solution, or just SSL) is certainly necessary, but it’s not sufficient, they only provide the "glue", whereas something like Dropbox provides a total solution. OTOH, if you want to share all kinds of resources and services, well beyond simple file and folder sharing, then something like Hamachi would probably make more sense.
At least that’s the way I view it.