As of version 0.4.3, Spotify includes the ability to add your own mp3 files to the music library. This is really good news - you no longer need Slocify to mix Spotify tracks and local tracks in the same playlist! However, I will keep Slocify on this page in case you want to use it anyway.
It'd be nice to see a new Slocify...
To say Slocify is no longer needed is ignorant. The best part of Slocify wasn't that you could play your library in Spotify. It was the fact you could play Spotify tracks in Winamp, using Winamp's superior i/o engine.
It'd be nice to have this working, since Spotify doesn't even let you change output, and I use other hardware for playback instead of my main soundcard.
Slocify is still available, I
Slocify is still available, I tested it with the latest versions of Spotify and Winamp just now. However, what you just described has unfortunately never been the features of Slocify. I built Slocify to be able to play both local and Spotify tracks in the same playlist (Slocify is a play on words with Spotify and local). With Slocify, Winamp is basically used as a remote control for Spotify and all Spotify music playing is being done in Spotify. What you are describing is really interesting but I think it would be hard to implement. However, Spotify has a public library (Premium only last time I looked) so it could be possible (for Premium users).