I keep seeing these widgets that show that “so and so just purchased this products X amount of time ago.”
Provely.io is a new competitor and has an opening sales for as little as $8/month while their grand opening sale is going on.
https://useproof.com/ appears to have more featues, but it's $29 a month and has a limit to the number of visitors
http/www.provely.io shows when people purchase, a few less features, and you can get it during their launch for $17/month ($8/month if you choose yearly). This also enables you to simulate purchases to get the ball rolling.