|EchoNest has created a Pandora Radio-like streaming radio feature for use in the Spotify app, and they're calling it Echofi. The Echofi experience is pretty smooth -- type in an artist, and it will launch Spotify and play music similar to that artist.
Andy Smith (who previously did Spotibot) created a browser-based app that uses Spotify to recommend artists in a continuous stream of music, in a similar manner to Pandora Radio. Earlier this Summer Spotify launched an API called libspotify 9, which opened the Spotify app for use in a variety of ways by outside developers. Consider it an open-source-like way of letting anyone and everyone create new Spotify apps.
Echofi creates a nice alternative to the normal Spotify playlist experience, by creating a streaming radio that runs on its own and brings a sense of music discovery. You only have to drive it as much as you want to, like Pandora. In a Spotify vs. Pandora comparison it's a little more clunky as it involves both your browser and the Spotify app to be open, whereas Pandora is contained into one item. Pandora also has its Music Genome recommendation engine, which has a long history of proven success. Echofi is a great start.
Spotify made it's U.S. debut in July.
Check out more about steaming music sites in the Spacelab Streaming Music Guide.