Public Domain

AntiqueClipart.com
Victorian, Edwardian and other vintage era clip art.
http://antiqueclipart.com/

Army Images
Collection of official Military, DoD and Army photos.
http://www4.army.mil/armyimages/

Bayonet.net
U.S. Army images and photos.
http://www.bayonet.net/clip_art/index.html

Clip Art Blog
Weblog featuring public domain images and related articles.
http://publicdomainclip-art.blogspot.com/

Clker.com
Searchable version of openclipart.org in ODG format. Allows direct import into openoffice.org and API for direct embed into weblogs.
http://www.clker.com

Dictionary Images
Collection of expired copyright images from Webster's New International Dictionary of the English Language, 1911.
http://srufaculty.sru.edu/david.dailey/public/public_domain.htm

Free Photos for Commercial Use
Categorized collection of photographs available for public and commercial use.
http://public-photo.net/

FreePhotos.se
Personal photography collection placed in the public domain by the photographer.
http://www.freephotos.se/

Ireland First Clip Art Archive
Collection of Irish-themed clip art without restrictions on use.
http://www.eirefirst.com/clipart.html

Jampot
A collection of public domain clipart and graphical resources.
http://www.jampot.eu

Logo Design Web
Categorized collection of stock photography released as public domain.
http://www.logodesignweb.com/stockphoto/

Nicu's Clipart Collection
Categorized clip art in SVG format released by the author as public domain.
http://clipart.nicubunu.ro

Ninja-AC
Ninja illustrations and Ninja graphic materials in JPEG and EPS format free for commercial use.
http://www.ninja-ac.com/

Open Clip Art Library
Archive of user contributed clip art that can be freely used.
http://www.openclipart.org/

Public Domain Clip Art
25,000+ public domain clip art images. Categorized and searchable.
http://www.pdclipart.org

U.S. Environmental Protection Agency
Gallery of environment-related images not subject to copyright restriction.
http://www.epa.gov/newsroom/pictures.htm

U.S. Fish and Wildlife Service
Photographs and drawings of fish, animals and outdoor scenes.
http://images.fws.gov/

U.S. Fish and Wildlife Service - Line Art
Line drawings of wildlife from the US Fish and Wildlife Service.
http://www.fws.gov/r9extaff/drawings/drawing.html

U.S. National Parks Service Digital Image Archive
Images of national parks, monuments and historic sites released to the public domain.
http://photo.itc.nps.gov/storage/images/

WP Clipart
Line art and other drawings scaled down for use in documents for typical home or small office printing.
http://www.wpclipart.com/

Zorger Public Domain Images
Collection of book illustrations and photos.
http://public-domain.zorger.com