The Finder. I like the ability to connect to other computer not in the LAN with out having to set up all this other crap. I am constantly needing things of the MP at home and that will just save me from so many headaches.
Spaces.
I've recently been using VirtueDesktops which is great but its development was halted on the announcement of spaces before fixing the bugs. Looking forward to a complete implementation of multiple desktops "with drag and drop ease", lol, obviously been dragged in by Apple's marketing ploy.