Buy any two regularly priced Angry Birds Tee Shirts, and choose a free shirt from four designs. They are:
Line Up - http://www.angrybirdstees.com/store/angry-birds-mens-t-shirts/line-up.html
Angry Ammo - http://www.angrybirdstees.com/store/angry-birds-mens-t-shirts/angry-ammo.html
Tough Guy - http://www.angrybirdstees.com/store/angry-birds-mens-t-shirts/angry-birds-tough-guy-mens.html
Super Cool - http://www.angrybirdstees.com/store/angry-birds-girls-t-shirts/super-coolness.html
Not entirely clear which of the four are available in what sizes/styles. If you go to the main website, the "All" page doesn't show three of the four shirts. I had to go to Men (2nd & 3rd) and Girls (4th) to find the links to the free shirts.
Not clear if this will work with items other than Tees.
They are also offering free shipping.
I've bought shirts here before, and the quality has been pretty good.