IMPORTANT - It is recommended that you sign up or use service to use properly licensed audio for your content. There are both paid and free options. Some of the assets listed below are just links to playlists, or websites that have web players. Naaackers personal recommendation is that you stay away from playlists or YouTube channels you do not manage, or do not have their licenses/agreements listed. There are obviously still some fantastic free options like Pretzel, StreamBeats, and BackingTrack. But these are platforms managed by creators who have blanket licenses for EVERY track on their platform. And they will handle issues and license questions on your behalf. You never know when someone else can add a track they don't have a license for to a Spotify playlist and screw you. Be careful out there!
There are others who have created additional resource lists for creators looking for DMCA-Safe music. If you'd like to view them, please see Other DMCA-Safe Resources