Yellow pages - okay, so they're not yellow - contain links to travel oriented sites worldwide.

Associations and organisations Accommodation
International Hotel and Restaurants Association (IH&RA)

Aeronautics and aviation
Air Transport Action Group (ATAG)
Air Transport Association (ATA)
Association of Air Medical Services (AAMS)
Aviation Home Page
Federal Aviation Administration (FAA)
Helicopter Association International (HAI)
International Air Cargo Association (IACA)
International Air Transport Association (IATA)
International Civil Aviation Organisation (ICAO)
Royal Aeronautical Society (RAeS)
World Airlines Clubs Association (WACA)

Conference and incentive travel
Association for Conferences and Events (ACE)
International Association of Conference Centers (IACC)
International Association of Convention and Visitor Bureaus (IACVB)
International Society of Meeting Planners (ISMP)
Society of Incentive and Travel Executives (SITE)

Travel and travel agents
Adventure Travel Society (ATS)
American Society of Travel Agents (ASTA)
Association of Corporate Travel Executives (ACTE)
International Airline Passengers Association (IAPA)

World and regional (general)
Pacific Asia Travel Association (PATA)
World Tourism Organisation (WTO)

Computer reservation systems
Amadeus
Galileo (Apollo)
Worldspan

Financial
Charge and credit card institutions
American Express
Barclaycard
MasterCard International
Visa
Currency converters
Interactive Currency Table

Fun and interesting
Flags of the world
Online converters
The why files

Health and Medical
Air Safety Home Page
Healthtouch
International Travellers Clinic
International Vaccine Institute

Time and weather
CNN weather
Meteo-France
USA Today - weather guide
World Climate: weather, rainfall and temperature data