The Gmail PHP SDK by Google currently in Beta stage includes authentication and authorization with Api Keys and OAuth 2 to start project development. Additional resources include building the client object, building the service object, calling an API, and handling the result.
Guerilla Mail is a simple idea with one purpose: to avoid spam. The idea is dead simple: go to the website, it gives you a random e-mail address that's good for one hour, and links to nothing else. Register for sites or whatever with it, and then never worry about it again. Although the web service is convenient, sometimes it's not everything you need. For those cases, one might want to look into using the Guerilla Mail API.
When it comes to Social Networks, Facebook definitely towers over the rest when it comes to numbers. While Facebook might be hard at work to expand its reach by entering new areas, Mxit which is Africa’s largest social network has plans of its own to expand its base by throwing open its platform via the Mxit API and encouraging developers to integrate Mxit into their applications.