Photos and Videos
Pexels: You can modify works and do not have to give credit to the original creator. You cannot use works for commercial reasons. Read the Pexels license for more information.
Videvo: In the Filter menu, choose the checkboxes for "Free" in the Clip Types section, and "Creative Commons 3.0" and "Videvo Attribution License" in the License Type section. Videvo also lets you know if video clips are safe to use in YouTube videos.
OpenVerse: Contains a mix of Creative Commons and public domain photos.
FreePik: Contains a mix of photos, icons, and videos. Most have a CC-BY ShareAlike license (always double check!).
Picnoi: Photo library featuring people of color as the main subjects. These photos have a CC-BY license. Read about Picnoi's license information.
Icons
The Noun Project: A mix of public domain and Creative Commons icons and photos you can use in your projects. Double check for what type of license the artist has given to their work.
Reshot: A library of icons and illustrations for free download. IMPORTANT: The site says you don't have to give credit to the icons' artists, but always double check the website's license information.
Music
SoundCloud: Some music tracks are licensed under a CC-BY-NoDerivs or a CC-BY-Noncommercial-ShareAlike license. Look at the music's detail page to check what permissions the artist has given for that particular piece of music.
FilmMusic: Find music for various genres to fit your project. Tracks are licensed under a CC-BY license.
Free Music Archive: Find music from various projects to fit your project. You can search for music based on CC license.