The KiSSmetrics Ruby SDK by Kissmetrics allows API communication to implement marketing optimization features into applications. It aims to engage users and analyze behaviors for customization purposes. In the usage section, developers can access methods with an Api Key.
Expion, a Social Media Management System (SMMS) company, specializes in helping companies manage their brand images on social networks, from Facebook to Youtube to Twitter, often relying on partners' APIs to pull of some of their more interesting feats. Most recently, it was chosen by Google as a Google Plus API partner for the brand-focused Pages feature.