Have you ever noticed that when surfing around the web you’ll often see big banner ads to websites you’ve recently visited? That’s due to this company—[AdRoll](https://www.adroll.com). AdRoll-enabled sites set a cookie when you visit, allowing AdRoll to later display banner ads from those same sites when you visit _other_ sites. (The notion is that repeat exposure is instrumental to conversion.)
In my own experience though, I most often see AdRoll ads for services _I’m already subscribed to_; obviously a waste of impression.