I don't have a White Castle near me, else I'd probably try for this promotion. White Castle is celebrating it's 90th birthday by giving away a special 90th birthday pin at their stores. But you need a coupon to get it (print from here). There is a limited supply (only 90 per store) and if you happen to snag one, be sure to wear it between July 25 and August 31 to receive a free Original Slider with your purchase. If you go to your store and present your coupon, but they're out of pins, you'll still get a free Original Slider.
Check out their press release for more information.