slyrics is an external lyrics addon for Spotify.
It uses Spotify's undocumented local HTTP API to retrieve info about the track that is currently playing. You can learn more about this API by reading Carl Byström's blog post. It then scrapes various websites to find lyrics and displays it to the user.
The goal is to be similar to Lyricfier in terms of functionality. The main difference is the fact that slyrics uses GTK for its GUI, while Lyricfier uses electron. The functionality of this application is far too simple to be able to justify running a separate instance of Chromium for it.
Currently, only Linux is supported. It shouldn't be too hard to port this to other operating systems, but it is not considered a priority.