This list is incomplete but is eventually intended to link to lots of DMV websites (where you'll often find sample plates available for sale, sometimes customized, usually for $20 or less).


...and of course as soon as I started to compile this information I ran across a site that proves someone has already done it for me. Yay! (I haven't explored the whole thing, but cool!)
Thanks, Melsh's Plate-O-Rama at http://home.centurytel.net/melsh/LP.html


Alabama
Alaska
Arizona
Arkansas
California
Colorado
Connecticuit
Deleware
Florida
Georgia
Hawaii
Idaho
Illinois
Indiana
Iowa
Kansas
Kentucky
Louisiana
Maine
Maryland
Massachucetts
Michigan
Minnesota
Missippi
Missouri
Montana
Nebraska
Nevada
New Hampshire
New Jersey
New Mexico
New York
North Carolina
North Dakota
Ohio
Oklahoma
Oregon
Pennsylvania
Rhode Island
South Carolina
South Dakota
Tennessee
Texas
Utah
Vermont
Virginia
Washington (state)
West Virginia
Wisconsin
Wyoming
D.C.
American Samoa
Federated States of Micronesia
Guam
Marshall Islands
Northern Mariana Islands
Palau
Puerto Rico
Virgin Islands

Alberta Alberta government services page
British Columbia licensing company history of BC sample plates
Manitoba
New Brunswick
Newfoundland and Labrador
Northwest Territories
Nova Scotia
Nunavut
Ontario
Prince Edward Island
Quebec
Saskatchewan
Yukon