Beets is an open source commandline-based music organisation tool.
OpenWhyd is an open source, platform-independent website and app for collecting music and creating playlists.
Smarter Playlists by Paul Lamere is an online service for rules-based generation of music playlists.
Spiff Radio is a platform-independent website for browsing and sharing playlists. It’s built with the open source WP SoundSystem plugin for Wordpress, and the proprietary SoundSystem API.
Tracker is a open source system service that provides simple indexing and searching of music for the GNOME desktop.
The Hype Machine scrapes the web to create playlists of recommended music.
Radiooooo is a paid service that lets you generate playlists by country and decade.
Roon provides proprietary software to organise and stream music around your home, with a cloud-based recommendations engine.
Spotify funds a lot of [research into playlist generation](https://research.atspotify.com/).
Swarm.fm connects with Spotify to produce recommendations of new releases by artists you listen to, and other things.
A selection of relevant papers. There are many more.
“Scaling Up Music Playlist Generation”, Aucouturier and Pachet
Uses adaptive search to select music according to user-defined constraints.
“Fast Generation of Optimal Music Playlists using Local Search”, Pauws, Verhaegh, and Vossen, 2006
Uses simulated annealing to select music according to user-defined constraints.
“Automatic Playlist Generation from Self-Organizing Music Map”, Hartono and Yoshitake, 2013
Uses a self-organising map based on musical features
“Automated Generation of Music Playlists: Survey and Experiments”, Bonnin and Jannach, 2015