Tuna is an OBS plugin that grabs information from various music players and puts it into a text file. You can use this text file as a source in OBS.
The Scroll Filter in OBS is useful for fitting longer song titles onto the screen.
A number of artists generously permit their music to be used on Twitch, and in some cases on Youtube and elsewhere. Most require either the purchase of their music, or a music streaming service subscription. Please check and make sure to abide by any requirements and restrictions if you decide to use their music.
Ben Prunty's Bandcamp Ben Prunty composes original music and game soundtracks. He permits his original work to be used for streaming, provided that it is credited and linked. For details, refer to his blog.
Karl Casey @ White Bat Audio Karl Casey creates synthwave, horror, rock and metal tunes that are free to use for streaming and Youtube, provided that you comply with the license requirements . Direct download is available on Karl Casey's website, no purchase is required.
Duett - sparkly, nostalgic chillsynth. See this Tweet for the permission.
Home - warm, dreamy chillsynth. See this Tweet for permission. Please be aware that tracks from this artist have been falsely claimed by a music company that doesn't have the rights to distribute Home's music. There is more info on how to dispute those false takedowns in the Twitter thread.
Gab Manette - optimistic, nostalgic chillsynth. See this Tweet for permission.
FIXT Music Program - this program requires sign-up, but it's free and allows me to stream music from a range of artists as long as they are credited. Requires Spotify or purchase of the music.
Nightride FM Stream Safe Music Listing - a listing of Dark Synth, Outrun, and Synthwave artists who allow their music - either their entire discography, or parts of it - to be used on Twitch. Note that conditions vary, and sometimes you need to get permission from the artist.
Peritune - A library of Creative Commons music, frequently updated. Their site is in Japanese, but Google Translate works nicely. Requires credit to use.
Open Game Art - A place to find music and other assets covered by a variety of licenses, including CC0. Licenses vary.