I still use foobar2000. The library/metadata management it is quite proud of is entirely built on your existing folder structure (it does not mutate it; it auto-explores/watches it for changes) and your ID3 tags (which it only mutates on your explicit edits). That was a big deal at the time it launched, especially versus how Windows Media Player and iTunes both fought to control your folder structure, rearranged everything form time to time, and would update your ID3 tags at the whims of Microsoft's or Apple's "genius" tagging databases (which were often full of mistakes, especially for rare/weird files).

Foobar2000 was the breath of fresh air opposite of "software/vendor lock-in" and a model that software today still often fails to live up to.