There's SO MANY MODS. Many of the old ones are still kicking, some were adopted, some by the original developers. If you plan on doing a heavily moded gameplay, I'd suggest looking into optimization guides tho, some mods that used to be a must have back in the day now have better, performance-friendly options.
Keygen Church is SO GOOD. I really dig this fusion of metal, glitch (I guess? I don't really know a good term for it lol) and baroque music. I'll check Igorrr out, I've been craving something in that vein for the longest time!