One of the biggest consolidator who provide "name your own price" opaque search, which is airlines and flight schedules are not disclosed before your purchase. It also provide fixed-price offerings now.
Another opaque search website which is owned by Expedia. It offers consolidator tickets.
For opaque search or so called "bidding", you should check Bidding For Travel and Better Bidding message board to see others successful bidding price as reference. Read their FAQs before you start bidding at Priceline. Remember, if you win the "bid", you can not modify or cancel your itinerary.
One of the largest USA domestic consolidator, it search both its consolidator prices and published fare.
One of the top airline consolidators in the country, 1800CheapSeats specializes in last-minute airfares and leisure and corporate travel services. Best for super-flexible travelers.
Lowest fare is previous owned by former CEO of TWA, now it is operated by priceline.
The least-known of the major online domestic USA consolidators. it check both published and consolidator prices.