Official Casio Sites:

GShock.com – Official G-Shock site
Baby-G.com – Official Baby-G site
G-Shock.jp – Official G-Shock Japan site
G-Shock.eu – Official G-Shock Europe site
World G-Shock – Other official G-Shock sites
Official Pro Trek USA
Official Pro Trek Asia

Official Casio Support:

Casio USA Support
Casio Europe Support
Casio Malaysia Support
Casio Singapore Support

G-Shock Replacement Parts (USA):

PacParts
American Perfit

G-Shock Online Shopping Sites:

Amazon.com – Authorized Casio reseller for watches labeled “sold by Amazon.com”
Macys.com – Authorized Casio reseller
ShopCasio.com – Official Casio mail order site