Wikipedia (free)
Wikipedia is a compendium of human knowledge, edited by anyone who cares to contribute. It has articles covering everything people might want to know about, including references to source material for further reading. Its goal is to become an authoritative encyclopedia.
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: