I found this one while buying magazines instore. I was told by the shop assistant that it was only an online deal but the leaflet I picked up says you can fill out the form and post it instore.
When you sign up you get £20 worth of vouchers via email when you join and an additional £10 of vouchers every month. It's a great one if you shop in WHSmith quite a lot. The vouchers also last longer than the normal instore vouchers.