Windows/Mac (and Android/iPhone/BlackBerry): SugarSync's detailed desktop-to-mobile-and-back syncing service is upping the ante in the cloud storage market, offering free users of its service 5GB of online space for synced files across an unlimited number of devices.
The thumbnail difference between SugarSync and the, well, more-publicised Dropbox service is that SugarSync is a bit more hands-on, but therefore also customisable. You can sync most any folder on your system, and choose to have only certain file types synced from those folders, too. You can also just use the service in a one-folder, Dropbox-like fashion, but you'll find it more useful for keeping track of documents scattered across your computer. The free accounts are available for anyone who signs up, and the upgrade is also free for existing users.
5 GB Free File Storage [SugarSync]