A2Z Traveler bills itself as "The Ultimate Travel-Only Search Engine and Directory" and they are well on their way to becoming that. When we visited, they had accumulated "more than 65,000 of the very best travel websites from around the world." in their directory, which is supplemented with results from the DMOZ project.
Searching for travel-related goods and services has become one of the principal uses of the Internet and, considering that volume, it's logical that travel-only search engines and directories would serve that market. A2Z Traveler does that very well.