There are many images online that are in the public domain - copyright free!

You can always take pictures from any site that is a .gov site so in addition to the sites below, you can always do a .gov search. For example: if you want a public domain image of a search warrant, in Google Image Search, type this: search warrant site:.gov and find the results that are only from .gov sites.

Public Images (General)

Photo Pin
The MorgueFile - Great for backgrounds, beautiful, professional images - searchable - use SHIFT+Command/Apple+4 to take screen shots of the photos
http://edupic.net/ (Not public domain, open specifically for education)
PD Photo
Public Domain Images - wide variety of topics, organized by category
Pics4Learning (search for Wolfe as the photographer in Advanced search for government-related images)
Public Domain Pictures.net
OpenPhoto
Free Pixels
FreePhotosBank
Free Stock Photos
Pictures from the US Government #1
Pictures from the US Government #2
Uncle Sam's Photos
Portrait Gallery

Specifically US History: http://www.multimedialearning.org/FreeImageArchive.php

Public Domain Clip Art
PDClipart
Public Domain Clip Art blog
http://etc.usf.edu/clipart/ (Open specifically for education)

Public Domain Music & Sound
If you are creating a music video, be sure to use music that copyright clear - either you own, one from Animoto that is free, or music downloaded from one of the sources below. DO NOT use a copyright-protected song. If you have questions about this, see Ms. W - don't guess.
Royalty Free Music (Great for Remix Video soundtracks :)
Free Classical Music by Composer (Click on the desired composer, then "view" then you can download it.
Free Music Now
Public Domain 4U
Live Music Archive (you can browse the artists, scroll down, click in the mp3 column to preview the songs. If you find one you like, back up one screen, right click [control+click MAC] and save as an MP3 onto your desktop. )
Music and Arts Internet Archive (pretty much same process as the Live Music Archive)
Free Sound Project - Set up a free account (no spam, I promise) and you have access to the most amazing sound library!
http://www.soundzabound.com/
More options for Copyright Friendly Music - Be sure to read the licensing information before you download.

Philosophers & Documents
Philosopher & Document Images in the Public Domain
http://www.constitution.org/cs_image.htm
http://www.epistemelinks.com/Main/ImageSources.aspx

Magna Carta
http://www.archives.gov/exhibits/featured_documents/magna_carta/

Mayflower Compact
http://www.historyplace.com/unitedstates/revolution/revgfx/may-compact.jpg

Portrait Gallery: http://www.lib.utexas.edu/exhibits/portraits/index.html

Philosophers & Documents
Philosopher & Document Images in the Public Domain
http://www.constitution.org/cs_image.htm
http://www.epistemelinks.com/Main/ImageSources.aspx

Magna Carta
http://www.archives.gov/exhibits/featured_documents/magna_carta/

Mayflower Compact
http://www.historyplace.com/unitedstates/revolution/revgfx/may-compact.jpg

Declaration of Independence & Constitution - See the Philosopher & Document Images links