Get basically all stock data, including historical data, via JSON API. Free. Free redistribution. Etc.

API designers, please note the following rules before calling your creation a REST API:

The Tin Can API (sometimes known as the Experience API or xAPI) is a brand new specification for learning technology that makes it possible to collect data about the wide range of experiences a person has (online and offline). This API captures data in a consistent format about a person or group’s activities from many technologies. Very different systems are able to securely communicate by capturing and sharing this stream of activities using Tin Can’s simple vocabulary.

play around with Google's API's using OAuth. see how the request/responses interactions should take place.