trushuffle

truShuffle is a client for Spotify that fills your playlist with similar music, based on what's currently playing.

It is available as a desktop client, supporting all major operating systems. If you download and install the client on your desktop computer, you no longer have to scrobble your tracks to last.Fm or have your browser open! Additionally, the tracks will be added after 10 seconds of playing the same track.

There's something called "scrobble", and it's a function that exists in Spotify. While "scrobbling", you share information about your listenings to a site called lastFm. Being logged in here or using the client, is pretty much all you have to do.