Popular streaming music service Spotify, which recently launched in the US, has added iOS support for its libspotify API. With the API, developers can add Spotify streaming to practically any iPhone or iPad app.

Spotify's API makes it possible to embed a player into any app that allows Spotify users to log in and stream music from the service. Prior versions of its API were available for Windows, Mac OS X, and Linux, but the latest update brings support for iOS as well.

The APIs are freely accessible to anyone who wants to use them for non-commercial use. However, apps that cost money or even earn revenue via advertising must make an arrangement with Spotify to use the APIs. We imagine this restriction may limit the number of developers that consider adding support, but for some developers, licensing the API will make sense.

The question remains, however, what kinds of apps users would want to have Spotify integration? GigaOM posited that games that let you play music could offer Spotify as an option. The first thing that came to our mind were training and fitness apps such as RunKeeper or Cyclemeter. Or how about an app that plays the exact opposite of what you want? We imagine that along with Spotify's metadata API, developers will come up with some new ways for users to discover and listen to music.