There are only two places I have been able to use Passbook with so far: Duane Reade and Starbucks.
Duane Reade (Owned by Walgreens)
Earn rewards without having to carry around those pesky Duane Reade rewards cards. Download the Walgreens App to get started.
Earn rewards and pay your bill using the Starbucks passbook. Download the regular Starbucks app and link your Starbucks Card to it.