Hacker News Reader

A public example is rutracker.org. Another is sci-hub. See Joe Karaganis for some cannonical info: http://piracy.americanassembly.org/shadow-libraries/

There's also The Internet Archive that provides torrents that's in the clearnet.

Music, movies, tv, video games, books. Some trackers specialize in one type of content, some have everything.

Finding URLs of private trackers is trivial in google. Getting an invite usually requires knowing another user, unless they temporarily open signups for everyone.

Here's one of many lists of private trackers

https://hdvinnie.github.io/Private-Trackers-Spreadsheet/