You can order 5 stickers for free on their site - http://www.concrete5.org/marketplace/swag/.
If you put a concrete5 sticker on something and send us a picture, you get an easy 500 Karma points. To make this even more appetizing right now, we'll mail you a sticker for free. Just shoot us your mailing address.
And yes, they do ship worldwide. So get your stickers now!
What I've done is, put a sticker up on my iPad's back. I always take my iPad (with a keyboard case) to clients, so I can easily write down notes and let them see responsive sites in action (in both landscape and portrait mode). With this sticker, I've got another legit reason to bring it with me: to promote concrete5. Since my keyboard case is already pitchblack, this sticker looks really nicely integrated. Awesome.
I've yet to think about what I will be doing with the remaining stickers, but at least 1 of them is used for a great purpose already. Care to let me know what you have in mind? Write a comment below!