Upbid

Upbid is a single public leaderboard. Anyone can submit a link and bid to hold a position on it; the highest bids sit at the top. The whole product is one page, one mechanic, and a payment flow that has to be exactly right.

It is the smallest complete product on this list, and the one that answers a question fastest: will people pay for attention in a market where almost nothing is priced that way.