www.conservingnow.com. It's removable, double sided, and Free! I'm usually pretty good at remembering to bring my bags when I shop, but I'm hoping that seeing the sticker out of the corner of my eye when closing the car door will help with those times that I'm too busy or distracted to remember on my own. I chose the rectangular tree design, but they also have a round one with an image of the earth.
*Please excuse the terrible webcam pic of the sticker :)