One application that provides an application API to access vehicle data and even control certain things in the vehicle is nobdy. nobdy works by exposing a dbus interface to applications for access to common vehicle services often found in modern vehicles. Through this API, applications can gain access to controls like door locking and extract information such as velocity, fuel economy, etc. The vehicle service API is very much a work in progress and may be extended to meet future needs.
nobdy also exposes a lower level API where applications can "subscribe" to specific bus messages.
nobdy provides a 3rd party plugin API so that OEMs can create vehicle-specific "providers" for their vehicles. This also puts the burden of security on the OEM to protect sensitive bus messages.