Gaming Sites:

Holmes Basic forum on OD&D Discussion - highest traffic forum dedicated to Holmes Basic & my favorite place to discuss anything from this site

Classic D&D forum on Dragonsfoot
- high traffic but covers all forms of Classic D&D

Holmes Basic forum on Knights & Knaves Alehouse
- dedicated to Holmes Basic but low traffic

Classic D&D forum on OSRgaming - med
ium traffic, covers all forms of Classic D&D

Holmes Basic page at RPG Geek - has a forum for posting

Collecting Sites:

The Acaeum - high traffic
General Collecting (TSR) Forums
- occasional Holmes Basic-related discussion

Tome of Treasures - low traffic
TSR Borderlands Keep forum - general TSR collecting discussion
Holmes Basic research forum
Basic D&D modules research forum
Games Workshop editions research forum - includes UK Holmes Basic