Expedia.com is a good place to start looking for flights. Its comprehensive network of travel agents and airlines is valuable for booking flights and for packaging flights with hotels and car rentals. Start with Expedia, and if you aren't sure about the prices of flights, visit the air carrier's website and try to find the same flights for less.
Though Flight Center also offers package vacations, its specialty is finding cheap flights. It has weekly deals listed on its main page, and also has an exhaustive search program. It's a good bet that Flight Center will be the cheapest option for main routes, and it backs this claim with a "Lowest Airfare Guarantee" that promises to beat any "qualified" airfare by $10.
The only shortcoming of mainstream travel companies like Expedia and Flight Center is that they tend to search only the big carriers and ignore cheap, regional carriers that might fly routes more frequently and cheaper than the big names. Search on flylowcostairlines.org for the more obscure carriers, and see if you can't beat the prices listed on the other two sites.
