lululemon has done well for itself by giving away its own branded reusable bags with purchases. I see these walking billboards for the brand all the time throughout Toronto.
But they don't confine giving them away to their physical locations, they even include them with online orders. During checkout you have the option to "bag me" or not. The inclusion of the thumbnail of the bag is a nice visual touch that subtly makes the user feel as though they're getting a bonus with their order. I also like that their playful use of language extends into the checkout process.
Thanks to Russell for the tip. No thanks to Russell for the picture of him in tight lululemon pants that's now stuck in my head.