Aluminum foil. Use a gluestick to put it over your bedroom windows. Now there is darkness, and you can sleep. This made a huge quality-of-life difference to me, and I felt very silly for not doing it 10 years earlier. (A sleep mask, which I previously used, was not nearly as good a solution.)
I've found tinfoil often lets through entirely too much sunlight (the thin cheap stuff, evidently). But I did do something similar with black card in one house. Now we use blackout curtains, so we can get light through the windows when we actually want it.
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: