Have you ever seen an Android developer give out promo codes before? Most likely not, because Google has only just added the feature. A poster on Android Forums has spotted a page on Google's Developer Console with instructions on how devs can create codes and how they can get their apps ready to use them. Based on the details written on that page, the codes can be used to purchase apps and pay for in-app goodies, but not for in-app subscriptions.
Update: Google has officially announced the program, check out all the details here.