Our API directory now includes 39 affiliate APIs. The newest is the Omnistar API. The most popular, in terms of mashups, is the Best Buy BBYOpen Products API. We list 8 Best Buy BBYOpen Products mashups. Below you'll find some more stats from the directory, including the entire list of affiliate APIs.
Lately we're seeing a lot more talk about API monetization. Last week we covered TweetPhoto's efforts at encouraging developers to use the TweetPhoto API through a transactional payment model (one penny or so for each photo uploaded via the API). Last month we covered Bandsintown, which provides affiliate earnings via use of its Bandsintown API. And more recently Touchnote announced an affiliate payment model (with a whopping 30% royalty) for developers working with its image API.