Smarter Playlists
I've just pushed out an early alpha version of Smarter Playlists, my summer spare-time project. Smarter Playlists is a playlist builder that lets you create new interesting playlists by combining sources of tracks (like albums, artists, other playlists) and filtering, sorting and re-arranging these tracks into a playlist. For example. Here's a simple playlist that combines two Spotify playlists: Morning Commute and Your favorite Coffee house into a single new playlist:
Here's a more complex example that starts with a Gothic Metal playlist and mixes in the top tracks from the band Ravenscry into the first songs in the playlist.
This is still very much an alpha version, so there are likely to be a bug or two - but give it a go if you are so inclined.
Check it out at Smarter Playlists