There are many great license plate websites. Here are some of this site's favorites:

Reference Website URL / Link
ALPCA - Automobile License Plate Collectors Association
Andrew Pang
Bruce Bufalini
Chuck Sakryd
David Nicholson
Eric Tanner - Detailed License Plate Information for All 50 States
Eric Taylor - Encyclopedia for Porcelain License Plates
Jeff Francis - License Plates For Sale
Jim Moini
Jordan Irazabal - Pictures of Low-Number Delaware License Plates - High Number Trackers
Mike Coleman & Terry Hammer - License Plates For Sale
Mike Glauboch - License Plates For Sale
Mike Sells - License Plates For Sale
Norm Ratcliffe - State Trooper Plates
PennDOT Driver & Vehicle Services Updates
Philadelphia Free Library Digital Collection
Rick Kretschmer - Info on Pennsylvania License Plates, Plus MD, VA, and More!
Timothy O'Connor - License Plate Collection
Worthpoint - Archive of Past Auctions and eBay Sales
Y2K License Plates - Huge Archive of License Plates