Apple has added a great new feature to Mac OS X Leopard but it's only available to .Mac members. Back to My Mac is a remote access tool that uses .Mac to locate your other Macs over the Internet and display a list of them in the Finder. From there you can connect, access files and even open up a remote screen.
Obviously the technology used to do all of this is nothing new: VPN connections have been around for a long time, VNC has also been around for a long time; it's free and runs on just about any platform. Don't forget about Windows remote Desktop. What is new about Back to My Mac is the way various technologies are tied together in to an ‘easy to use' almost seamless manner.
I haven't upgraded to Leopard yet but I am a member of .Mac-a service which really works well in synchronising personal data between multiple Macs. Back to My Mac may well give me an excuse to upgrade!