The travel agencies listed here represent some of the most popular ones on the web. They not only have online reservation and ticketing services for air, car, hotel, cruises, rail and vacation packages, but most have destination travel information as well.  Unless otherwise noted, they cover both leisure and business travelers. Most sites are in the United States. A few are in the United Kingdom. These are indicated by the British flag.
 

Web Travel Agency Ratings

• ConsumerWebWatch  The Consumer WebWatch researches web sites, including travel sites, and evaluates their information. It is a project of Consumer Union. They have a report on travel under the Research Reports section which you can download as a pdf document.
   
• ConsumerSearch-Travel Sites  Their Travel Sites review is excellent.  They also mention the other sources of travel site evaluations.  Here is a link to their travel site comparison chart.
   
• DMOZ Travel Agent Associations.  Comprehensive list of worldwide travel agency associations.
   
• Links2Travel - Lists travel agencies of the World by country.