The Playlist Miner is a web app that creates Spotify playlists based upon matching keywords.
Give The Playlist Miner a keyword or two like "workout" or "happy" and The Playlist Miner will find as many Spotify playlists that match those keywords. It will then aggregate all of the songs across all of these playlists and find the top 100 songs that occur the most frequently in the matching playlist. These top songs can then be saved to your own playlist.
The Playlist Miner is online at http://static.echonest.com/playlistminer/index.html
It makes use of the Spotify Web API.