rsync
Mike Bombich has written a tutorial on using rsync over ssh (via Mark Pilgrim). This is an amazing combination. I can change a few files locally and synchronize (or, as purists would say, reconcile) the entire ATPM Web site (10K files) with the server in a few seconds. This is my favorite example of something that Mac OS X can do right out of the box, but that was hard even with third-party software on OS 9.
