Google Images (click tools -> usage rights -> creative commons)
Everystockphoto - Search engine for free images.
Open Clip Art - Lots of free clip art.
Pics 4 Learning - Free copyright friendly images for educational use.
List of Public Domain Image Resources - Even more copyright friendly sites
ccMixter - Thousands of free sounds and music to download and use.
Piano Society - Classical piano music uploaded by users that is free to use.
YouTube Audio Library - Download background music for your video, completely free.
Jamendo - An independent music community with unlimited free streaming and downloading.
Free Music Archive - Many genres available as well as curated sounds to help you find what you need.
Melody Loops - Royalty free background music.
SoundBible - Free sound clips, sound bites and sound effects.
JamStudio - Create your own music online and download it. Registration required.
Netlabels - Part of the Internet Archive. The collection is completely free with catalogs of virtual record labels.
Partners in Rhyme - Some pay and some free content. Also has effects, Midi and loops to use.
Moby Gratis - Created by the artist, Moby, Online application system, which registers you to use the site.
Incompetech - Background music to download by Kevin MacLeod.
Sound Search Engine - Huge site for finding sound effects.
Classic Cat - Find classical music for video soundtracks.
Purple Planet - Lots of free full length songs to download and use.
Zap Splat - Free sound effects with a huge variety to choose from.
PD Sounds - Public Domain sound library.
Cylinder Preservation and Digitization Project - Mostly Public Domain songs from before 1924 that have been digitized .
Soungle - Royalty free sound effect search engine.