Spotlight (Tiger Review)
The best overview I've seen of Spotlight thus far. The bottom line -- it works well enough to be useful, but it has at least two more revisions to go before it will be a solid tool. I'd guess 10.4.4. (10.4.1 is out, 10.4.2 will focus on bug fixes, 10.4.3 will start to address functional issues).
Using Spotlight with application controlled data stores is the interesting question (Entourage, FileMaker, etc). This is much more ambitious than merely indexing documents. I dimly recall old issues of BYTE referring to a 1970s era OS that had a true database as its file store -- it could manage this trick.