The PayChannel API makes it possible to incorporate its credit card processing gateway into your software platform. Public information on the API is not available, though it does provide 8 payment features.
Read-it-later service Instapaper has released what it's calling a "full API." With it, developers using the Instapaper API can create applications that both write to and read from the service. The full API is free to developers, but only paying users can read from their Instapaper account using third-party tools.