Below are a few links that (in no particular order), in my opinion, are well worth visiting!

General Interest Sites:
Fallen Leaves
Model Horse $ales Pages (Carie Sapp)
The Watering Hole (Amanda Zins/Leesa Miller)

Sculpting/Painting:
Rose Studios (Sarah Rose & Todd Speaker)
Rio Rondo (Carol Williams)
Chinook Studios (Melanie Miller)
Aspen Leaf Studios, LLC (Jennifer L. Irwin)

Painting:
Grand Cheval Studios (Steph Michaud)
Beau Cheveaux (Chris Nandell)
Grey Willow (Kristi Sawin)
Pony Painter Studios (Tracy Eilers)
Elk's Bugle (Melissa Mistretta)
Stvdio Spirans (Daphne Mazuz)

Hobbyists:
Red Devil Stables (Teresa Fedak)
Colorado Hobbyist Yahoo Group

Competing:
North American Model Horse Shows Association
Model Equine Photo Showers Association

Retailers & Casters:
Mountain View Studios, Inc. (Sheri & Will Rhodes)
Resins by Randy (Deb & Randy Buckler)
PS Cord & Co. (Sandy & Paul Cord)
The Lakeshore Collection, Ltd. (Cindy Neuhaus)

Automotive
Grassroots Motorsports
Sports Car Club of America
Winding Road (e-zine)