Dropbox now has 8GB 10GB for free with a few referrals! When did that happen!
Use this link to get +250MB when you signup
I use it to sync files between my Macbook, Windows 7 PC and work computer. It also has an iPhone app.
It works by not only storing your files in the cloud (for online access) but by also storing the file locally on all devices with Dropbox installed.
Combined with programs that work across both platforms like Keepass (http://keepass.info/) and Xmind (http://www.xmind.net/) it’s a great tool to always have your files available.