Travel may be down in this economy, but there's one area where business is booming: discount booking sites. With travel providers desperate to unload unused rooms and tickets, it seems like the only sectors of the travel industry that are exploding are the ones previously seen as a last resort by the industry.

Priceline.com reported that bookings through its site, which offers steep discounts on hotel rooms and other travel products, are at their best levels in nine years. Hotwire.com, which is owned by Expedia, reports a similar boost in revenue. The Hotwire Group's sales over 12 months have exceeded $250 million for the first time, and reps told WalletPop that compared to last year, hotel bookings in the biggest markets (meaning the most popular travelers' cities) are up between 20% and 30%.It sounds really rosy, but of course, there's a more ominous reason for the big numbers: Hotel suppliers have lots of empty hotel rooms to get rid of, and they would rather take a piddly amount for those rooms than allow them to go empty. In fact, 4.5- and 5-star hotels, which once avoided the brand taint of dealing with the big discount sites, are now flocking to Hotwire to unload their "excess capacity." This year, Hotwire added some 4,000 more properties, all eager to fill space however they can.

Both sites offer huge discounts on hotels -- it wouldn't be uncommon to pay $50 for a room that was formerly $200 in San Francisco or Fort Lauderdale -- but to get the best deals, customers usually don't know which property they're getting until they've accepted the price and the general neighborhood. When times were plusher, many travelers scoffed at using the so-called "opaque" booking sites, fearing they'd get burned. Now that money's tight, many complaints have fallen away.

It's still easy to get a lemon on those sites. Proceed without caution, and you could wind up with an airfare requiring two stopovers or a fleabag hotel room. But those instances seem to be decreasing, partly because so much higher-end stock is flooding the options. With conventions drying up thanks in part to the "AIG Effect," business-class hotels are increasingly represented on the blind discount sites, so as long as bidders request properties of a high star rating, they're extremely unlikely to get the shaft.

For prime strategies for using these "opaque" bookings sites to your advantage, including links to several sites that travelers use to gossip about which hotels you'll get for lowball prices on those sites, check out my list of ways to game Priceline and Hotwire.