When you buy your plane tickets online, there are basically three categories of websites you might want to check out.

1. Airlines: You can buy your airline tickets directly from the airlines. Some airlines offer fares available only on their websites.
Airtran Airways
Alaska Airlines
American Airlines
Continental Airlines
Delta Air Lines
Frontier Airlines
Hawaiian Airlines
JetBlue Airways
Midwest Airlines
Northwest Airlines
Southwest Airlines
Spirit Airlines
United Airlines
US Airways
Virgin America
more airlines
  
2. Aggregators: Aggregators search multiple airlines and redirect you to airlines website for booking. In return, aggregators usually receive commissions when you click on the search result. Some airlines may not participate in aggregator's fare search system.
Bing Travel
Fly.com
ITA Software
Kayak
Momondo
Mobissimo
Skyscanner
WeGo
  
3. Online Travel Agencies: Similar to aggregators but offer booking service. These might be useful when you need hotels and cars in addition to a flight.
Expedia
Orbitz
Priceline
Travelocity
 

 

Airfare Price Change (historical data): click here

airfare price change chart

Cheap Plane Tickets to US Domestic Destinations:
Travel through December 15, 2010 -- from AirTran
Book your tickets by August 24, 2010, at 11:59 pm ET. Travel is valid through December 15, 2010. 10-day advance purchase required. Lowest sale fares are valid for travel on Mondays, Tuesdays, Wednesdays, Thursdays and Saturdays. Blackout dates are as follows: November 23,24,27,28 and 29, 2010.
Across the US -- from American
Purchase tickets at least 10 days prior to departure, or within 1 day of making reservations, whichever comes first, but no later than 11:59 p.m. (CT) August 24, 2010. Travel through December 15, 2010. Travel Monday through Thursday and Saturday; other days are subject to higher fares. Embargo Dates: November 20, 23-24, 27, 29, 2010. Min/Max Stay: Up to a Saturday-night minimum stay may be required, based on travel days.
Atlanta & Minneapolis flights to select U.S. cities -- from Delta
Tickets must be purchased at least 10 days prior to departure, but no later than August 24, 2010. Travel must be completed by December 15, 2010. Blackout Dates: November 23, 24, 27, 28, and 29, 2010. Fares shown are valid for travel on Mondays, Tuesdays, Wednesdays, Thursdays, and Saturdays. Higher fares are available on Fridays and Sundays. Minimum Stay: Saturday night. Maximum Stay: 30 days or through December 15, 2010, whichever is earliest.
Northeast to Florida -- from Southwest Airlines
Purchase by September 1, 2010, 11:59 pm Pacific Time. Travel September 7 through November 15, 2010. 14-day advance purchase required. Travel valid only on Tuesdays, Wednesdays & Saturdays.
Unlimited travel from September 7 to October 6, 2010 -- from JetBlue
$699 for 30 days of unlimited travel. $499 for 30 days of unlimited travel (excludes Fridays and Sundays). Pass travel valid on JetBlue-operated flights in the JetBlue route network only. Domestic taxes and fees included. International and Puerto Rico taxes and fees not included. Travel dates: Tuesday, September 7, 2010 through Wednesday, October 6, 2010. Each flight must be booked and ticketed no later than 11:59 p.m. EDT or 11:59 p.m. local time, whichever is earlier, three (3) days prior to the flight's scheduled departure.
Fares from $49 each way -- from Virgin America
Requires a 7-day advance purchase. Tickets are non-refundable and non-transferable. Tickets must be purchased by 11:59pm PDT August 23, 2010. Travel must occur between August 23, 2010 and November 17, 2010. Service to/from Orlando (MCO) begins October 6, 2010.
 

Cheap Airline Tickets to International Destinations:
Canada
Europe
Mexico, Caribbean, Central & South America
Asia & Oceania
Middle East & Africa