Mac OS X only: Free preference pane application Fruux syncs Address Book, iCal, and tasks between different Macs. To use it, just install the preference pane and create a new account with Fruux. Once you've verified your account, go ahead and run your first synchronisation. Fruux uploads your contact and calendar information to the cloud so it's ready to sync to any of your other Macs. Just wash, rinse, and repeat with as many computers as you want to sync with. Fruux is smart, too, supporting sync conflict resolution when a record has been changed on both computers. You can already roll your own contact syncing with Address Book and Google Contacts or push contacts, and Google Calendar syncs with iCal without too much effort, but if you'd prefer a more streamlined alternative, Fruux provides a dead simple install-it-and-forget-it syncing setup.
Even better, the Fruux roadmap reveals more ambitious goals, including Safari bookmark syncing and—more importantly—preferences syncing. Essentially, then, Fruux is aiming toward building a homegrown MobileMe. If this app remains free, it's got crazy potential written all over it. Fruux is a free download, Mac OS X only.