A while back, Burger King was offering free french fries to spread the word about their newer, healthier (less unhealthy?) fries. I'm not sure if this onion rings freebie is along the same lines, but you can get your free "value-sized" onion rings in participating Burger Kings this weekend, Feb. 4-5.
It doesn't look like a coupon or purchase is necessary; the only restriction is that you can't get them during breakfast hours. Not that you'd want onion rings for breakfast anyway... would you?