General Information

Acme Pet Site (lots of information here)
American Shetland Sheepdog Association
American Kennel Club
OFA (Shelties listed by quarter)
VetGen

Breeders

Carmylie Shelties 
Classy Shelties
Foxglove Shetland Sheepdogs
Istari Shelties
LinLayn Shelties
Mary Jo Sminkey's Dogpatch
Oakdale Shelties
Okie's Shetland Sheepdogs
Rapporlee Shetland Sheepdogs
Romayne Shetland Sheepdogs
Ross Roi Shelties
Shiloh Shelties
Springmist Shetland Sheepdogs
Sue Ann Bowling's Site (ROM shelties with pedigrees and background, history, size charts, genetics including color, national winners, national results, much other sheltie stuff).

Publications

Sheltie International
Sheltie Pacesetter