Inspired by the meteoric rise of Groupon, a 2-year-old group buying site that pitches daily deals in more than 500 cities worldwide, more than two dozen companies are now vying for a spot in vacationers' inboxes. They offer a limited number of rooms for a restricted time (typically a week or less) at a small, rotating collection of hotels, resorts, condos and smattering of cruise lines. Most bookings are non-refundable and non-transferable. In return, they promise flexible, opportunistic travelers steeper discounts than deal aggregators like Travelzoo and Sherman's Travel and agency sites like Expedia, Orbitz and Travelocity, but without the guessing game of booking through Priceline or Hotwire. (Expedia offers its own daily "ASAP: A Sudden Amazing Price" flash sale, spotlighting two hotels at up to 50% off its regular rates.) Many flash-sale sites - Jetsetter, HauteLook Getaways, Rue La La, SniqueAway, Vacationist and Voyage Privé, among them - concentrate on high-end options and restrict access to members, though membership is free to anyone with an easy-to-land invitation or code. Others, like just-launched Overstock's Vacations, Trippo and Yuupon, cater to more mass-market tastes. Get the full story at USA TODAY