NeoFinder ($40, but the trial version will probably do what you want, for free)
I spent months trying to find a program that would keep an updated index of the files on my two (very large) NAS drives so I could search them as quickly as I search with Google. I tried almost a dozen programs and Mac hacks, and none of them worked even though several of them should have worked in theory. And then I found NeoFinder, and it worked perfectly. Now I can finally search my NAS drives without waiting 30 minutes for each search to finish.
Hopefully I have now saved at least one person several months of searching. :)
For windows, I use X1 Desktop search.
I have often benefited from recommendations for Things I Didn't Know I Wanted.
Given that Less Wrong is a community of unusually intelligent, critical, and self-improvement-focused people, I suspect we can generate a pretty helpful thread of product recommendations — perhaps even a monthly thread of product recommendations.
Rules: