Rdio. Music Streaming service. Not sure what happened after they closed up shop but the UI was beautiful and they had great performance.
I really miss Rdio! Just disagree on the performance a bit. The UI was fluid, but their network performance would take a hit from how their API had been designed: every action on the app would trigger a large JSON payload download (and sometimes upload). It definitely felt nicer than Spotify.
I was sad when I had to switch to Spotify. Rdio was clean. Even today I don't understand Spotify.
Rdio also had an amazing recommendation engine from Echo Nest. Even though that company got bought by Spotify it took them a long time after the acquisition before they had decent recommendations.