WaterPik water flosser. Flossing does more to improve oral health than brushing, but I had significant trouble keeping it up as a habit because it was awkward and required regularly replenishing a supply of floss. Water flossing appears to be about as beneficial as flossing with nylon but is far more convenient, and a tech you will use is better than one that you won't.
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: