All kinds of self-published stuff, lots of which later became commercial. You will have heard of some of it, for sure. Darude - Sandstorm? That was from there. DragonForce were big in the power metal category. The band that became Linkin Park came from there. And then hundreds of thousands of indie artists (including an earlier me).

The RIAA's action there destroyed vast amounts of music, pretty much the equivalent of if someone just aggressively deleted Bandcamp and Soundcloud put together and everything on it because they were upset they didn't control it all. I will never forgive them for that.

Another random band I was listening to because of MP3.com was Lazlo Bane which is best known because one of their songs "Superman" (which was on MP3.com) became the Scrubs TV show theme song.

MP3.com also had a bunch of very early meme bands such as The Laziest Men on Mars with songs based on "All Your Base Are Belong to Us" and "The Terrible Secret of Space" which were viral and hard to escape in certain friend groups.

Because most of the music was free to download and a lot of it was pretty viral, I'd maybe add a Spotify if it was Legal Napster analogy to the Bandcamp and Soundcloud put together analogy.