1X1 Call Sign Database  U.S. callsigns
Clover Operators Call Book - DX
Clover Operators Call Book - USA
FCC License Database  U.S. callsigns
FCC Callsign Database  U.S. callsigns, by University of Arkansas
HamQTH.com  by Petr, OK2CQR
IK4LZH  QSL Infosystem
ON6DP  Database
OZ7C  QSL Search Machine
Pathfinder Web Client  Search country specific call books
World Wide HamCall Search Service  by Buckmaster