MobileMe sounds worthwhile for syncing between multiple Mac devices, as Isa says, and it also gives you a simple way to create a personal website to share photos, blog, etc.
A free online option which provides some sync capability and redundant backup is Dropbox, getdropbox.com. I only have one computer, so the value of Dropbox to me is offsite, redundant storage of important files (I also use TimeMachine), but it would allow you to share certain files between home and office computers, for example, even if one is Mac and one is PC. It might be tricky to set it up to emulate MobileMe’s ability to sync AddressBook and so on. You can store up to 2 GB with a free account, and there are paid options for more.
Karen