iOS (Jailbroken): Siri is a fine assistant if you're using all of Apple's default software, but it doesn't support third-party apps. AssistantEnhancer is a jailbreak tweak that adds a bunch of new commands to Siri, including the ability to control third-party music apps including Spotify and Pandora.
AssistantEnhancer comes packed with new commands that allow you to search the app store, check storage space, toggle system settings, and search for local events. However, the most useful feature it adds is the ability to control songs on Spotify, Pandora and Grooveshark. You can say commands like "Play artist 'Bob Dylan' on Spotify" or "Start a station for Bob Dylan on Pandora". If you're looking to get a little more out of Siri, AssistantEnhancer is a good place to start.