Do you use MyPoints? I've used it for years, maybe a decade, and usually just click through the emails that give you five points for clicking. I've gotten lots of gift cards through the years as a result.
I second using MyPoints. (Thinking back on when I started using it....holy crap, 15 years?) The click thrus are nice, plus occasionally some vendors I buy from online will have deals such that I can get bonus MyPoints from purchases, which gives me a tiny thrill when it overlaps with bonus cash back on my credit card.
i-say.com has changed a couple times over the years; it's another marketing research company. I've made very modest amounts of money over the years, including some gift cards (not great bang for your buck, but it's nice to be able to just hop in every now and again and do a couple surveys, and eventually cash out for an amazon card or something).
I'm a long-addicted Diet Coke addict (yeah, I know), and have gotten a ridiculous number of gift cards from Coke Points over the years. Most of my points actually came from friends who didn't mind giving me their codes, or codes I snagged from the dumpster when I was taking out my recycling. I usually pick up a couple small ($10) amazon cards over the course of a year, but always make sure to save up a bunch for the holidays, because they always have a promotion with a bunch of "discounted" gift cards.