Spotify lets you do a great many things, but quickly assembling a playlist is not one of them. The playlist-creation process can be laborious, where you must add individual track after individual track. Sure, you can add entire albums to playlists, but who wants to do that?
With MagicPlaylist, you can create Spotify playlists in a snap, while potentially discovering new music along the way. MagicPlaylist is a Web app that uses an algorithm and the Spotify API to create playlists based on songs you give it.
Just head to the MagicPlaylist website, enter a song and hit Enter. You'll get a playlist based on that song. You can't listen to the tracks (other than 30-second snippets) on MagicPlaylist, but you can save playlists to Spotify for your listening pleasure. You can delete any unwanted tracks before saving the playlist to Spotify, but you can't reorder the list.
I found MagicPlaylist's results to be fairly compelling. I got a good mix of songs for the most part but would lodge one complaint: bands are repeated too frequently in a playlists on the whole. The worst example of this was when I plugged in "Blank Space" by Ryan Adams. The resulting playlist was basically a Ryan Adams vs. Ray LaMontagne showdown.
In related news, learn how to get a Spotify or Rdio playlist of your Shazams.