US Federal Government Public Domain Images - Primarily Photos
Databases of images from the US government--most are public domain
Selected Internet Resources --Government Resources for Science Images
Online image databases for science topics. These are government sources and therefore mostly public domain.
Musopen - Free Public Domain Classical Music
Sheet music and audio in the public domain. You can hear a random piece or listen to their online radio station too.
IMSLP/Petrucci Music Library: Free Public Domain Sheet Music
Public domain sheet music library. This is a wiki where contributors scan and upload scores.
NounProject
Black and white icons with ability to search. Some are public domain, some creative commons, so check the license for each one before downloading.
53+ Free Image Sources For Your Blog and Social Media Posts
Free image sources. Some CC, some public domain, some other licenses. Some sites require registration.
Pixabay - Free Images
Free public domain images
Open Peeps, Hand-Drawn Illustration Library
Public domain (CC0) Illustrated characters with mix and match hair, bodies, expressions, and poses.