AdRoll

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. 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.