DIY Zoning Project: Much to learn about design and construction of HVAC systems.
LifeTips: HVAC tips and explanations about equipment.
HVAC Talk: Message board discussing all aspects of HVAC.
Wikipedia: HVAC entry
Energy Star: The EPA's web site discussing Energy Star initiatives, products and news.
ASHRAE: American Society of Heating, Refrigerating and Air-Conditioning Engineers with a great Education section.