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 Canada Support
Casio UK Support
Casio Australia Support
Casio Europe Support
Casio Malaysia Support
Casio Singapore Support
Casio India Support
Other Countries

G-Shock Replacement Parts (USA):

PacParts
American Perfit