This site is a gem that has accompanied me on many spikes in the last year :) datasette's original music is top tier too. cognitively stimulating but not attention stealing.

Don't laugh, but for me, it's Abba. Their entire discography is ~3 hours which is how long I can maintain peak concentration. Their songs are consistently good so that I don't need to skip a song, but not too good that I would stop working and start listening. Plus I've never heard Abba song in any good movie so it doesn't remind me scenes from a movie I would want to rewatch. Of course I don't listen to it every day, only when I really need to, most daily programming tasks can be done with any music.

This sight got me through many projects in college :)

I recently discovered Lorn and have been mainlining his back catalogue ever since whilst working. Thoroughly interesting and immersive yet not distracting.