Bargain Travel Sites

1Travel
AirSaver
BestFares (subscription-based service)
Cheap Tickets Online
Hotwire - discounts on airline tickets
Lowestfare
Orbitz
Priceline
Smarter Living
TravelZoo - including Netfares Search engine

Travel Auction Sites

Better Bidding - Advice for using Priceline and Hotline
Bid 4 Cruise
Bid 4 Vacations
Priceline
RecommendedBidding For Travel - Online advice for using Priceline
RecommendedSky Auction
RecommendedThe Bidding Traveler  - More online help for using Priceline
Yahoo Travel Auction Listings
You Price It

Specialty Travel Sites

Cruise Critic
Cruise Mates
Elder Hikes - Specializing in travelers over 50
Europe Through the Back Door
Flyer Talk - Frequent Flyer Community
Single Parent Travel
Ski Trips
Solo Travel Network
Solo Traveller
Travel Chums
Travel Companion
Travelook - Specializing in gay and lesbian
Why Travel Alone
Wind Jammer - Sailing Cruises
Women Traveling

Cruises

Cruises, Best Prices & Service
The Cruise Ship Center
Charter in Paradise! Welcome to Aladdin Cruises!
Costa Cruises
TravelSource: Cruises
Dream Adventures
Caribbean and Cruise Travel Magazine
Young Family Travelers
Sunset Concert Cruises

Recommended = A site I have found to be very useful.