Known for it's great user interface & unique comparison engine, Hipmunk has firmly established itself as a go-to site for hotel & airline accommodations....View More They make money by earning referral fees for flights & hotels booked so it's in their best interest to make sure you find what you're looking for on their site. One of their clever marketing strategies to keep you glued into their site is to get your email address so they can update you on flights you're looking for & re-direct you back to the site when you're ready to buy. However, the opt-in form itself isn't the big takeaway away here. The big takeaway is the timing & context in which the pop-up appears. It's only while you're waiting for flights to load & essentially, doing nothing, when they ask you for your email. They include a progress bar & searching text to retain your attention & present you with a targeted offer of flight updates specifically related to the flights you're looking up. Underneath the form, they discreetly add a checkmark to email you outside of the updates.