spotify-qt - Lightweight Spotify client using Qt written in C++.
spotify-tui - A Spotify client for the terminal written in Rust.
Spotifyd also supports the Spotify Connect protocol which makes it show up as a device that can be controlled from the official clients. Spotifyd streams music just like the official client, but is more lightweight and supports more platforms.
Spotifyd - An open source Spotify client running as a UNIX daemon.
Spot - Gtk/Rust native Spotify client for the Gnome desktop.
Psst - Fast and multi-platform Spotify client, made in Rust with a native GUI.
ncspot - Cross-platform ncurses Spotify client written in Rust, inspired by ncmpc and the likes.
Mopidy - An alternative plug-in based implementation of Music Player Daemon is able to stream from Spotify with an extension.
MellowPlayer - A free, open source and cross-platform desktop application that runs web-based music streaming services in its own window and provides integration with your desktop.
It enables applications to use Spotify's service (streaming), without using the official closed-source libspotify.
Librespot - An open source client library for Spotify.
Note: All clients listed here require a Premium account to stream from Spotify.