APS (USA)
American philatelic society, the online portal for stamp collectors from around the world. over 50,000 members enjoy many benefits and services.
http://www.stamps.org/
ASCAT
Square internet- en softwarediensten
http://www.ascatonline.org/
ASDA (USA)
The american stamp dealers association: serving stamp collectors since 1914
http://www.asdaonline.com/
IFSDA
IFSDA
http://www.ifsda.org/
PTS (UK)
Welcome to the philatelic traders' society. this site is intended for use by both philatelic traders and collectors alike.
http://www.philatelic-traders-society.co.uk/