Fan sites:

Jag-en-linge - a fan site by Brigitte in French and English
Totally Steven - a fan site devoted to Steven Culp (Clayton Webb)
DJE Online - the unofficial site of actor David James Elliott (Harm)
The John M. Jackson Admiration Society
Randy Vasquez Online - a fan site dedicated to Randy Vasquez (Gunny)
Bud Lovers Anonymous - a fan site dedicated to Patrick Labyorteaux (Bud)
JMJ Online - a fan site devoted to actor John M. Jackson (AJ Chegwidden)

Message Boards:

JagNik FanFic - FanFiction and discussion
The Harmy Board - Spoilers and discussion

FanFiction Sites:

AeroGirl's JAG FanFiction
JAG FanFic by Chris Gomez
Charlestonian Romantic - Fan fiction by Belinda
JAG Smut Sisters - A fan fiction site featuring Stacey, Rita and Kay
Alekater's Fan Fiction - Fan fiction by Tammy
Jagged Hearts - Fan fiction by Sheri
Under the Covers - A JAG smut archive. This site is password protected.
Visit the JAG Virtual Series site!

Official Sites and Fan Clubs:

Catherine Bell Online - (Sarah Mackenzie)
Chuck Carrington Fan Club - (Jason Tiner)
Peter Murnik Online - (Clark Palmer)