I built this because I like listening to slowed music but searching for pre-made versions is a pain.
Moonlit grabs tracks from YouTube or TikTok using yt-dlp on the backend, then lets you adjust pitch, reverb, and playback speed in real-time in your browser with the Web Audio API and/or signalsmith-stretch.
Replace youtube.com or tiktok.com with moonlit.wastu.net in the URL. There's a userscript too if you want a button on those sites.
Features: pitch lock & shifting, reverb, lyrics support, WAV export, self-hostable with Docker.