The Internet has created a whole new world for air travelers. It has given you, the customer, access to the same computer systems that travel agents use (that's why travel agents always seem so sad nowadays . . . ). As a result, you have a lot of control over your time in the air. There are sites that will check the status of your plane, check fares for you, e-mail you when fares in your price range pop up, let you comparison shop, and allow you to buy tickets from the convenience of your home at 2 a.m.

The Internet is most often used as a research tool. Not everyone who finds their best rate online actually buys online, instead turning to the airline or a travel agent. But buying online is a handy option for those who want to get it done as quickly as possible, given that if you don't buy it now, the fare could be gone by tomorrow. Below is a review of some of the most popular Internet travel sites by category.

Trip planners

These sites have several services that slice and dice your flight inquiries in any number of ways. They can search one-way trips, round trips, each leg of the trip, by price, by date, by time, and by multiple airports. And they can bake a cherry pie. They can do it all. They typically also offer services to alert you to when low fares for desired routes become available or to examine baseline fares offered by airlines throughout the year. Travelocity.com has airplane seat maps for 13 airlines, in case you want to choose your seat too. Many trip planning sites also have hotel room finders and car rental options. Cheaptickets.com has a Fare Aware option that shows what other passengers paid for a certain trip at the same time last year. For the top 1,000 routes in the United States, the service shows the average price, average number of passengers, what a one-way trip costs, the airline that flies that route the most often, and the low-fare courier rate. While some sites do international flights too, others such as Yahoo! only search within the Continental US, Canada, Puerto Rico, and the U.S. Virgin Islands.

One other great thing about these Internet sites is that you can often find sweet deals at the last minute. If you want to fly on a whim for the weekend, this is really the best route to go.

Popular sites include:

At www.smartertravel.com, you can do all of the above but also sign up for a weekly email with cheap fares available for the coming weekend from airports you can access. Its flight search option shows all flights leaving the day you are interested in but when you click on the price, you are bounced over to www.expedia.com. It also advertises the latest travel bargains and ongoing airline sales.

Auction sites

These fall into two categories: the typical auction and reverse auctions. At an auction site, the seller (either a company or an individual) puts an item up for sale to the highest bidder. At reverse auctions, individuals specify the price they will pay for a seat and the airlines either agree to the price or not.

Reverse auction sites include www.priceline.com and www.expedia.com, a Microsoft-owned travel site that has a feature enabling customers to name their price. These sites are fantastic because you can choose your own price, and you might get the ticket. So all you do is sign on, say where you want to leave from and where you want to go, what dates you plan to travel on, and how much you're willing to pay. The drawbacks are that: 1) you have to put in your credit card number before you know about the exact times of flight, so you're stuck with whatever you get, and 2) you often have to fly at crazy hours. But if you're looking for cheap and you're flexible, this is a great way to go.

Regular auction sites include SkyAuction.com, which sells available airline tickets and vacation packages. While you can't punch in your destination of choice, the sellers may have tickets to the very Caribbean island you have been dreaming about all winter.

Airline sites

Finally, one way to use the Internet to check for prices is to go to the airline sites directly. All major airlines (and the smaller ones too) have their own web presence where you can book flights. Many airlines have lower fares that you can only get when you book online or when you buy an e-ticket (a paperless ticket). So don't overlook them when searching for fares. Use any search engine, such as www.yahoo.com or www.altavista.com, and enter the airline of your choice to get to its site. But let us warn you that the best deals usually come from auction sites, or consolidators, or special deals. Going to the airline itself should be a last resort.

While the Internet travel sites are unbeatable resources that enable you to see the range of available prices, there is no one site that can guarantee the lowest fare -- no matter what they advertise. A recent search for flights from Boston to Toronto, Canada, on the above sites turned up "lowest fares" ranging from $222 to $500 at a variety of times and airlines. And we're sure that if we surfed for a couple more hours, we could have found a lower rate. The moral of the story: the more time you spend researching, the lower the rate you'll get.

There are some drawbacks to using the Internet. One is that most sites want you to register with them, which means filling out a registration form and thinking up a password that you won't forget. If you can, use the same name and password for all the sites so that you don't forget. Another hassle is entering your pertinent information. There are zillions of boxes to be filled out, so be prepared for some busy work. Once you've registered, you'll probably get constantly bombarded with e-mails from those sites. Those are beyond annoying. A final drawback is the lack of precision in some of the searches. You may have specified you want to leave at 8 p.m. and the site will show you flights leaving at 6 a.m. This is because the 6 a.m. flight is available. But for those who want to keep all of their options open, it is a boon in the search for rock-bottom fares.