Not sure if it qualifies as a "hidden gem" in OS X itself, but the new Signature management in Preview, using the built in iSight to scan a handwritten sig off a sheet of paper, is an amazing time saver.
This article is not like a lifehacker “10 neato things to be more productive on your mac” article. It's specifically about using DTrace to get detailed statistics on processes making system calls.
// Number 8, bitesize.d, rocks.