The premier site for information on protecting cultural property and art theft is found in Holland (moderated by Ton Cremers) the  Museum Security Network

Saz Production's Research: Stolen Art Directory

Museum & Cultural Initiatives:
ICOM ; Illicit Traffic of Cultural Property 
Red List ; 100 Missing Objects (series)
UNESCO ; Legal Protection for Cultural Heritage

Law Enforcement:
Directory Officer.Com
How to report stolen art.

Art Detectives - Security Consultants:
UK: Art Management Group
USA: Robert Spiel & Associates
USA: Steve Keller & Associates
USA: Layne Consultants Int.
USA: ASIS
USA: UNITEL
USA: XG Consulting Group
Italy: Investigarte
Ireland: Niall Leinster Consultants
Germany: Artresearch-Kunstdetektei

Companies / Databases:
Art Loss Register
IFAR (Authentication)
INTERPOL CD-ROM
Saz Productions (That's us!)
Trace

Museum & Cultural Associations:
American Association of Museums
ICOM - ICMS
UNESCO links -Ministries of Culture

Academic Organizations:
UK: Illicit Antiquities Research Centre
UK: Institute of Law & Art
US: SAFE

Cultural Property Law:
US: USIA Cultural Property
US: Customs
US: Laws - FBI Stolen Art File
US: NPS Laws & Regulations
UK: Arts Policy - City University
INT: Archaeology, Antiquities, and the Law

Conservation:
Conservation on Line

Bibliographies:
Object ID by Robin Thornes.
Antiquities Theft by Laura Pope Robbins.
Archeological Looting by Hugh Jarvis.
Art Forgery by Jonathan Sazonoff