Robocopy built into Vista
OK, perhaps not news to everybody, but I was surprised to learn this yesterday.
Robocopy is a powerful file copying utility that's much more robust than COPY or XCOPY. It's especially useful for huge file copies, especially over flakey network connections. See http://en.wikipedia.org/wiki/Robocopy for details.
In fact, if you run XCOPY /? in Vista, the help text begins with this:
NOTE: Xcopy is now deprecated, please use Robocopy.