This was a big market a few years back, with products from Microsoft, Google, Yahoo, and others. I tried them all, and only Yahoo Desktop Search (X1) was worth the bother. Alas, it's gone now. Google Desktop Search is still around, but it doesn't treat folders with sufficient respect and it doesn't give me enough configuration options.
Microsoft will focus on migrating users to Vista search, and of course OS X has spotlight, so there's only the soon-t0-dwindle XP marketplace for Yahoo to work with. I presume they entered this market in the first place to try to get YDS users to use Yahoo search, but I doubt that was worth the bother. It's especially not worthwhile now that Google has won the search wars.
I may try the X1 product, though when I last tried it I was surprised to find that YDS was less buggy than X1!
Update: I'm using it now. It seems as good as always, but I think YDS might have allowed me to do network shares -- X1 doesn't without an upgrade. In any case I don't need this feature -- my network is mostly OS X.
Update 2: Actually, it's better than YDS. I don' t think YDS included Eudora indexing, but this version of X1 does. Nice!