PoolManual.com
Series of articles about pool maintenance procedures, chemicals, and equipment.
http://www.poolmanual.com/

PoolSpa
Information including a product directory and locator for pool service professionals, builders, spa dealers and retailers.
http://www.poolspa.com/

Hot Tub Information Central
Industry insider provides opinion and information about construction and purchase of hot tubs. Includes an active hot tub forum.
http://www.angelfire.com/mi2/hottubs/

Poolsearch
Articles and forums with advice. Directory of vendors.
http://www.poolsearch.org/

Pool & Spa Living Magazine
Information about buying and caring for swimming pools and hot tubs, and links to retailers in Canada and USA. Also sells subscription to its printed magazine.
http://www.poolspaliving.com/

Spababes
Features a care guide, glossary, heater, and switch and relay advice.
http://spababes.com/

Hot Tub Guide.com
Information on hot tubs and spas. Compare features and models from spa manufacturers.
http://www.hottubguide.com/

Pool Wizard
Addresses questions about pool water chemistry and safety, and swimming pool problems. Includes step by step guides and pool calculators.
http://www.havuz.org/

Backyard Bay
Maintenance, purchasing, and safety tips featuring Michael Miller from Backyard Bay Radio Show.
http://www.backyardbay.com/

Swimmingpool.com
Discusses types and constructions of backyard pools, with an online design feature, dealer lookup, financing information, and photo gallery.
http://www.swimmingpool.com/

The Finnish Sauna
Mihael Cankar explains the history and ancient Finnish culture of steam bathing, the principles of sauna building, health considerations, as well as a few local terms and expressions.
http://cankar.org/sauna/

How I Built My Own Swimming Pool
In-depth information about the option of building your own pool, with pitfalls, advantages, and cost comparisons.
http://www.howibuiltmyownpool.com/

Discount Pool and Spa Supplies Technical Reference
Includes tools to determine water volume and pump size requirements, chemical balance guide, problems and solutions with heaters, filters and pumps.
http://discount-pool-supplies.com/technical.php

Swimming Pools and Swim Spas
Guidance on swimming pool, spa, or hot tub design, construction, and maintenance.
http://swimming.about.com/od/swimmingpoolsandspas/

The Swimming Pool Operators and Owners Resources
Information about installation, water testing and analysis, and chemical safety, for owners and operators in the United Kingdom.
http://www.caromal.co.uk/