I use the MobileMe service from Apple. The 20 Gb iDisk is enough for syncing files between my several computers. It's mounted on the desktop of my various Macs and with the iDisk's cache, copying files is instantaneous and I can work on my files while offline. When I get back to an internet connection, it syncs. I can also access my iDisks using the webinterface or mount the iDisk in WebDAV on Windows.
Also MobileMe Sync my iPhone and my computers Calendars, Address Book, Keychains, Bookmarks, Dashboard Widgets, Dock Items, Mail Accounts, rules, signatures, smart mailboxes, Notes, Preferences (including serial numbers) and FTP favorites. I can use the me.com website to access my adress book, calendars, files and emails from any web browser.
Configuration is very simple (login, password, thick a checkbox) and require no software install.