Aerinfo.aero
An introduction to the various reservation systems used by airlines.
http://www.airinfo.aero/

Amadeus
Provides a computerized reservation system and other I.T. solutions to the travel industry.
http://www.amadeus.com/

Amadeus.net
Check flight availability online.
http://www.amadeus.net/

Business.com
A very comprehensive list of reservation systems worldwide.
http://www.business.com/directory/retail_and_consumer_services/ travel_and_tourism/reservations_and_ticketing/index.asp

Check My Trip
Check your Amadeus booking on the net.
https://www.checkmytrip.com/
http://www.amadeus.net/

Gabriel
An airline reservations system from SITA.
http://www.sita.aero/

Galileo
Galileo operates a booking system that airlines, travel suppliers and travel agencies around the world depend upon.
http://www.galileo.com/
http://www.travelport.com/

Galileo Viewtrip
View your Galileo PNR/booking online.
http://www.viewtrip.com/

MyTripandMore.com
Worldspan reservation access website.
https://mytripandmore.com/

Pegasus Solutions
Provider of an international hotel reservation system.
http://www.pegs.com/

Sabre
Booking solutions for the travel industry.
http://www.sabre.com/

The Air Traveler's Handbook
Detailed information about online reservation systems and flight schedules.
http://www-2.cs.cmu.edu/afs/cs/user/mkant/Public/Travel/html/online.html

The Air Traveler's Handbook
FAQs relating to online reservation systems including Easy Sabre, Travelshopper and OAG.
http://www-2.cs.cmu.edu/afs/cs.cmu.edu/user/mkant/public/travel/online.faq

The Practical Nomad
A detailed article about PNRs, the industry jargon for a booking or reservation.
http://hasbrouck.org/articles/PNR.html

TravelGlobe.Biz
Guide to the technology, software and services used by airlines, travel agencies, tour operators and hotels.
http://www.travelglobe.biz/

Travelport
Formerly Cendent travel distribution services.
http://www.travelport.com/
http://www.galileo.com/
http://www.worldspan.com/

Utell
Utell is the world's largest hotel reservation system.
http://www.utell.com/

VirtuallyThere.com
Check your Sabre booking here.
https://www.virtuallythere.com/

Worldspan
Worldspan's booking technology brings together travel buyers, sellers and managers from all around the globe.
http://www.worldspan.com/
http://www.travelport.com/