Pane tickets! Keep in mind that 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: These might be useful when you need hotels and cars in addition to a flight. Some online travel sites may have marketing arrangements with selected airlines and may be able to offer fares lower than airlines.
Cheaptickets
Expedia
Hotwire
Orbitz
Priceline
Travelocity
 

 

Airfare Price Change (historical data): click here

airfare price change chart

Cheap tickets to US Domestic destinations:
►Spring Flights, select US cities -- from Delta
Tickets must be purchased at least 14 days prior to departure but no later than March 22, 2010.. Travel may begin on or after April 12, 2010. Travel must be completed by May 27, 2010. Minimum Stay: Saturday night. Maximum Stay: 30 days or through May 27, 2010, whichever is earlier.
►Across The U.S. -- from American
Reservations are required. Tickets booked on AA.com must be purchased at least 14 days prior to departure, or within 1 day of making reservations, whichever comes first, but no later than 11:59 p.m. (CT) March 22, 2010. Travel April 12 through May 27, 2010. Min/Max Stay: A Saturday-night minimum stay is required.
►Swing into spring -- from United
A 14-day advance purchase is required. Travel must occur between April 12, 2010 and May 27, 2010. Travel is valid daily. A Saturday night minimum stay is required and a maximum stay of 30 days is permitted. Exceptions apply.
►Chicago spring special -- from United
A 14-day advance purchase is required. Travel must occur between April 7, 2010 and May 27, 2010. Travel is valid on Tuesday, Wednesday and Saturday. Travel on other days of the week may be higher. No minimum stay is required. Exceptions apply.
►Travel through November 16, 2010 -- from AirTran
Book your tickets by March 25, 2010,, at 11:59 pm ET. Travel to and from Florida, San Juan, Aruba, Montego Bay, and Cancun is valid April 6 through November 16, 2010. Travel to all other destinations is valid through November 16, 2010. 10-day advance purchase required. Blackout dates for all markets are as follows: April 2-3; May 28, 31; June 18-20, 25-27; July 2-3, 5, 9-11, 16-18, 23-25, 30-31; and August 1, 6-8, 2010.
►Maui and Kona from Oakland, Sacramento and San Jose -- Alaska Air
Purchase tickets by March 25, 2010. Travel is valid from April 25, 2010, to June 10, 2010. Oakland/Kona flights operate Tuesday, Thursday, and Sunday. San Jose/Kona flights operate Monday, Wednesday, Friday, and Saturday. San Jose/Maui flights operate Tuesday, Thursday, and Sunday. Sacramento/Maui flights operate daily.
►Between Amarillo, Lubbock, and McAllen, TX and select U.S. cities -- from Delta
Tickets must be purchased at least 21 days prior to departure, but no later than March 31, 2010.. For Amarillo, TX, travel may begin on or after June 10, 2010. All travel must be completed by August 31, 2010. Minimum Stay: Saturday night. Maximum Stay: 30 days or through August 31, 2010, whichever is earlier.
►New Service From Chicago And Dallas/Ft. Worth -- from American
Plane tickets must be purchased at least 14 days prior to departure, or within 1 day of making reservations, whichever comes first, but no later than 11:59 p.m. (CT) April 13, 2010.. Travel is valid April 6 through June 9, 2010. Travel Tuesday, Wednesday and Saturday; other days are subject to higher fares. Travel is not valid April 10, 2010. A 2-night minimum stay is required and a 30-day maximum stay is allowed.
Check out US Domestic Destinations...

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