For years I used Microsoft's free product, FolderShare, which is now being replaced with Windows Live Mesh. I was a big fan since I have computers in both Chicago and Boca Raton and like to keep the infomation identical on both computers. When traveling back and forth between locations, there is no need to copy or transfer files. Files were always synchronized within seconds. It was always assumed that Microsoft would eventually start charging for this "free" service. That hasn't happened yet. What's worse is they replaced the FolderShare product with Windows Live Mesh, which requires Windows 7! Because I support a number of legacy software products, it is necessary for me to keep at least one computer loaded with XP.
I was very sad to see this happen, however I found a great alternative...Dropbox. Dropbox not only syncs my computers, but also stores my files "in the cloud". It's free for up to 2GB of data. You get 50GB for about $100 a year. I gladly paid for the extra space. It gives me additional peace of mind knowing that I can access all my documents on line as well. In addition, it also keeps the deleted files and allows you to share folders. Best of all, Dropbox doesn't care if I'm syncing with a Mac, Windows 7 or Windows XP.