SugarSyncis a small application which runs on Mac or PC: it's 'faceless' (meaning it only appears in the menu bar). It includes file sharing and a 'Web Archive' for storing files you don't also want to be on your Mac. You can select any folders on your Mac to sync.
It does block-level incremental uploading which makes uploading changed files a very quick process. Changes to the folders are automatically detected while the program is running and uploaded - a remarkably quick process as only the individual blocks of a file which has changed are uploaded. Another computer can then access SugarSync and download the files to the equivalent folders, providing quick syncing.
It doesn't have a background process: you leave the application running (it appears in the menu bar) - the Preferences give you the choice of having it run automatically at startup. Unlike some of the other methods available, if you quit it, it really is quit (if it is running an upload at the time it will automatically resume the next time you boot it).
There is a free 5GB plan: paid plans start at $4.99 per month ($49.99 p.a.) for 30GB. It's received good reviews: I've been using it and found it's flexible and easy to use; it would be my choice of the available methods.