We've reviewed five popular online storage services. If you want to be able to create, store, and edit documents online, consider using Google Docs or Windows SkyDrive. If synchronization is what you want, you'll probably go for Dropbox, Box.net, or Windows Live Mesh.
If you want both online document support and synchronization, consider using Google Docs or Windows SkyDrive and downloading a separate cloud client, such as Gladinet, to get the file synchronization. That arrangement would also give you better backup functionality and Windows integration.