Last winter, something unprecedented happened in the travel industry. People with reservations for peak holiday weeks at popular Caribbean resorts looked over their finances -- and canceled their plans.

The resorts had to do something with all those newly available rooms, so they turned to online auction companies, which put the rooms out for bid at prices well below retail. That surge in auction listings set off a wave of tremendous deals for savvy travelers.

The trend has only gained strength recently. Luxury Link, which specializes in high-end online vacation auctions, saw its listings jump 91 percent in the second quarter of 2009 compared with the same period a year before. Meanwhile, Sky Auction, which lists trips in a broader range of prices, says its number of auctions has been growing steadily too, with its sharpest increase also in luxury offerings.

So how do you get involved? Vacation auctions work like any other kind of online auction: A trip is listed on the site, and travelers bid on it until a preset end date, at which point the highest offer wins. In many cases, picking dates and booking is then left to you.