An electric toothbrush. Your teeth will feel cleaner, and apparently these toothbrushes have been "proven" to do a better job. I now dislike using the normal toothbrushes, eg when travelling.
I am currently using this one, bought for $49 not $199 on Groupon, cleans great and wireless recharging is great, but seems to have a design fault - I only managed to replace the head by wrecking the old one, will replace with a different brand.
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: