iOS/Android: From the same team that brought you SoundHound, Hound is a music search app that lets you speak the name of a song or artist aloud to search for it in the SoundHound database. Once it's found, you can read more about it, listen to a preview, or watch the video.
Unlike the previously mentioned SoundHound, which uses a song's audio pattern to identify the track playing, Hound allows you to just speak the title or the artist to see songs that match your search. When the app finds the song you're looking for, you can listen to an audio preview of the track, read more about the song or the artist, or hop over to YouTube to watch the music video.
If your voice search doesn't work, you can always use the keyword search. Hound is meant to be a song search tool, not so much a music-on-demand app. It does, however, give you an easy way to learn more about any artist you hear about, and preview their music. Hound is free, and available in the iTunes App Store and the Android App Market.