I've not seen any pop-unders but I heavily filter my internet. I remember back a few years ago Marriott, Hilton, and others used to have Roomkey.com pop-unders, but not sure if they're still using those or not.

Good: å∂bl0çk Plus: http://www.adblockplus.org (FT censored the name, so I had to get creative)
Better: Brave web browser: http://www.brave.com
Best: Pi-Hole: http://pi-hole.net