Which travel sites really offer the best values? The short answer is that it depends on the kind of trip you're planning. Some sites are better for finding a cheap, last-minute getaway to a sun-kissed Florida beach, while others are best for planning a visit to Madrid four months from now. The following guide to on-line travel features five common travel scenarios, along with the best websites for booking each one.

The Basics

The major sites:

There are hundreds of places where you can buy plane tickets and other travel services on-line, but a handful get most of the action. The big three — www.expedia.com, www.travelocity.com, and www.orbitz.com — are essentially all-purpose travel agencies. For any given trip, it's impossible to know which will offer you the most appealing flight, fare, or package, so always compare at least two of them.

Know less, pay less:

The next most popular sites are www.priceline.com and www.hotwire.com, both of which feature discounted fares with "opaque" pricing — meaning that customers are not informed of the airline or exact flying times until after they make the purchase. On Priceline, you name the price you're willing to pay for a given route on a given day, and if the site finds you the fare you want, you're committed to buying. Hotwire tells you the price of the ticket, and you decide if you want to buy it — and only get details if you do.

Airline sites:

Most airlines have easy-to-use sites that display many (but not necessarily all) of their best fares, sometimes offering perks like seating upgrades and discount coupons.

One-stop shopping:

In addition to buying plane tickets on-line, you can purchase travel packages that can include hotel and car rental. You can also use the sites to create custom packages — referred to as "dynamic packaging."

When to pounce:

Patience is key in the on-line travel game, because it can sometimes take a while to find a flight that fits your needs. But "if you find something you like and it's in your budget, buy it, "advises travel industry analyst Henry Harteveldt. "Chances are the price won't go down. It will go up."

Why not just call a travel agent?

The more complex a trip — the more stops involved, places to stay, things to do — the more likely you'll need the expertise of a good travel agent. They may have access to deals just as good as those on the Web. But when using a travel agent, be sure to inquire about basic service fees (usually $20) and any other booking fees.

Scenario: I know exactly where I want to go and when I want to go there.

Strategy:
1. To establish a point of reference, start with the powerful search engine at Orbitz.
2. Check those results against Travelocity, which is known for its value fares. (A Consumer Reports study last year found that, of the big three travel sites, Travelocity generally offered the lowest fares for the most convenient routes.)
3. Visit the websites of a few of the airlines that fly the route you're interested in, especially if they include low-fare carriers like JetBlue and Southwest. This advice applies to almost any scenario.
Trial Run: Philadelphia to Las Vegas. A weekend of gambling and shows in Vegas, then on to the natural splendors of southern Utah's national parks.
Result: On both Orbitz and Travelocity, the cost of a Continental flight with one stop was $292 round-trip. (Travelocity also offered good deals on convenient hotels.) A search of Continental's site turned up no flights at that price with equally convenient arrival and departure times.

Scenario: The destination is decided, but my timing is flexible. I can go away almost any week at any time — but no red-eyes, please.

Strategy:
1. Again, compare at least two of the three major sites — all of which offer special tools to search for the lowest fares over a certain period. The Flex Search feature on Orbitz lets you compare the lowest available fares over a 30-day period. Travelocity's main search tool lets you perform a similar task, and Expedia offers its Fare Compare (good for popular routes) and Fare Calendar functions. Even if it turns out you don't want to fly when the absolute lowest fares are being offered, at least you'll have a benchmark for evaluating other fares.
2. Given your flexibility, you should also check out Hotwire and Priceline. The trick to "checking" prices on Priceline without having to buy is to put in an absurdly low figure — say, $20. The site will then send back a note with what are, in effect, bidding recommendations: $X is a likely success; $Y stands a fair chance. Priceline claims the system usually doesn't respond to improbably low bids. So if you try this and it doesn't work, you'll have to wait till the end of the search, make a low but reasonable bid, and see what happens.
3. Other sites to consider: www.cheaptickets.com, www.lowestfare.com.
Trial Run: L.A. to Victoria, British Columbia. A summer week enjoying the rustic charms and ocean air around Vancouver Island.
Result: Expedia and Orbitz both had an appealing flight on Air Alaska for $294 round-trip, with one stop. Cheap Tickets offered a slightly cheaper ticket, but the departure and connection times weren't as convenient. A check of Air Canada's website turned up a winner: a fare of $289, with agreeable connection times. Using Priceline, in this case, wasn't worth the risk of having to travel at an inopportune time.

Scenario: Get me out of here, now! Send me anywhere, and take care of everything.

Strategy:
1. Most of the major sites offer lots of attractive last-minute travel packages. Check some of the major sites, as well as discounters such as Lowestfare.com, and the package-only vendor www.site59.com, a favorite among travel aficionados.
2. Cross-check the available hotels at a travel-rating site, such as Fodor's.
3. Depending on your flexibility, you might want to try Priceline and Hotwire, too.
Trial Run: New York to the Caribbean. It's Friday, it's 10 degrees, and I want to be on a warm, sunny beach this time next week.
Result: Site59 offered two tempting six-day air-and-hotel packages for Antigua: one for $903 (for "a quiet, unpretentious hotel") and one for $1,515 (for a luxury resort). Hotwire had a good plane fare for Antigua but no hotels. Combining the Hotwire fare with a hotel deal from Hotels.com exceeded the cost of Site59's special package, so that was clearly a great deal.

Scenario: I like my regular airline, but how do I know its website has the best fare?

Strategy:
1. Start with the airline's site. In some cases, this may be your only option. For example, popular low-cost carrier Southwest doesn't let the on-line travel agencies offer its flights.
2. Check a couple of the big sites and some of the discount sites to see how they stack up against the airline's. (Keep in mind that, while you can accumulate frequent-flyer miles on most flights booked through these sites, you can redeem them for travel only on the airlines' sites.)
Trial Run: Seattle to Tucson. A long weekend with family, without "wasting" frequent-flier miles.
Result: On America West's site, there was a round-trip fare of $245, with one stop. Orbitz had the same flight and same fare, but you have to pay Orbitz a $5 service fee.

Scenario: I want to go somewhere and do something off the beaten path.

Strategy:
1. Start with the major travel websites, but recognize that they're not as focused on unusual itineraries.
2. Look for firms (on- and off-line) that specialize in the kind of trip you're looking for. Try a good search engine, like Google. Or, even better, visit a site like www.about.com, which rates other sites.
Trial Run: Miami to Kenya. Find a safari tour (with a reputable company) that will take in the grasslands of the Masai Mara National Reserve.
Result: There were no safaris in Kenya on Expedia, Travelocity, or Orbitz, but a Google search turned up www.iexplore.com, which listed more than 20 possible trips from reputable operators. A nine-day safari sponsored by iExplore itself was fetchingly priced at $2,999.