Exit intent overlays
I've seen quite a few sites experimenting with services like Bounce Exchange (http://bounceexchange.com/), Exit Monitor (http://exitmonitor.com/) or one of the free, open source alternatives (https://github.com/carlsednaoui/ouibounce). These services try to detect when a visitor is about to leave your site (usually the moment they move their mouse cursor out of the viewport of the browser in the direction of the address bar) and show an overlay in an effort to try to get visitors to stay, usually by offering some incentive (think voucher codes, free shipping, free online courses etc).
It's easy to get this type of format wrong and add an overlay that is both disruptive to the UX and spammy-looking. But if done right (hence, by offering added value) it can decrease the bounce rate on your landing pages.
The only one of those that I've used on a live site is Exit Monitor. Their script is easy to integrate, works well and resulted in an uplift in conversions. Bounce Exchange are a full service provider (so, they'll build and design the overlays for your), Exit Monitor has a nice visual editor that helps if you're not a developer. The open source options will require a developer to work on the implementation, but seem relatively straight-forward too.
I'd say the exit intent is one of the most powerful triggers conversion-wise. I've personally tested out exit intent popups VS other triggers (such as after X seconds, %X page scroll etc) and in most cases exit intent wins big time.