A quick summary worth sharing courtesy of Emily @ Spotify:

A number of the sites share playlists with friends (Spotify Friends).

Some find random playlists (ShareMyPlaylists.com, Your Spotify, Spotify Playlists, xpandify, etc).

And some create playlists based on an artist you may be interested in. ie: if you really liked Keane and wanted to find similar music to them - these sites create playlists off the back of this artist: (Spotibot, Spotiseek).

There are a few more unusual ones - Spotify DJ (broadcasting DJ sets)

Topsify (creates playlists based on the charts, and historic number 1s)

Radiofy (essentially this site lets you pick your favourite radio stations, it then shows you what that station just played, and if you want to hear that song again or whatever you can just click on it and it opens in Spotify. To get past the opening screen and into the site click on ‘Stäng’. It’s in Swedish.)

And for even more services, go here.