As noted by 9to5mac, Apple has pushed an update to the App Store that highlights apps that support Passbook. These apps, which range from sports like the MLB app, to even the Walgreens app, hook in to Passbook to utilize the various functions. For instance, you can buy tickets for a baseball game and send them to Passbook.
Personally, I am skeptical of Passbook: it seems that Apple wants to get in to the payment market, yet they seem unwilling to dive all the way in with NFC and other methods of portable payment.
However, this initial push of Passbook-friendly apps does look interesting – while I still consider Passbook to be hampered by its reliance on QR codes, this initial push shows the backing of Apple and even third parties, which is crucial to a service such as this. If this support is a precursor of what is to come, I can see Passbook becoming more and more useful, perhaps even accomplishing the goal of eliminating the wallet for many uses.