We say it time and time again: you'll save money if you use a flexible date search. Choose to fly on the days and during the months when the airlines aren't as busy, and they'll give you a bargain. Yes, it's hard to believe that before airline deregulation, there were just 4 classes of fares--Y coach, F first class, YN night coach, and FN first class night--and it didn't matter when you flew. But now, it does matter when you fly, because the airlines are free to practice what they call yield management (charge higher fares for high demand travel dates, low fares for low demand ones). Usually, traveling on a Tuesday or Wednesday will yield the lowest fares (that doesn't mean that buying fares on a Tuesday or Wednesday will save you money.)
So here are some examples. All of these fares were found within a few minutes of each other on the same day and on the same airlines, using Travelocity's flexible date search and Travelocity's exact date search. Exact dates were chosen at random.
To learn more about flexible date search, please see our handy chart.
Example 1: Providence to Las Vegas in August
$236.50 including taxes using flexible date search)

or $369 RT including taxes not using flexible date search

Example 2: Santa Ana/Orange County to Orlando in August
$253.50 RT including taxes using flexible date search

or $356 RT including taxes not using flexible date search

Example 3: Atlanta to Denver in August/September
$286 RT including taxes using flexible date search

or Atlanta to Denver $451 RT including taxes not using flexible date search

Anyway, one thing I didn't notice in the chart is United [ual.com] isn't listed with it's flexible +/- 2 days search. Although most airline sites have it like NWA [3 days] and Delta [1], it's important to note some of them can actually pull up more expensive fares using flexible. NWA seems to be the worse IMO, because consistently when using the flexible search, once I click on a date I get a completely different set of flights [usually much fewer and more expensive] than if I just searched using the dates without using flexible first. It's quite frustrating! This has always been a problem with NWA, even after they "upgraded" to "Orbitz technology".
For example, on the first trip I took with them, I had to manually add MEM as a layover between ORD-DTW-XNA to find a cheaper ticket. I had seen the rate on another web site, but no matter what I did on NWA I couldn't bring it up. Once I did a multi-city itinerary schedule search and manually added the new flights, NWA "discovered" a lower ticket price! I'm sure all the web sites have their demons, but NWA needs to really overhaul theirs especially since they want us to book with them directly.
Finally, I've noticed United's search is great except when trying to find 2+ stops using flexible. They simply don't show up, unless you search by price [not schedule] and sometimes even have to click the 2+ stops to have their computer system search for cheaper inventory. I've also found this to be true with Orbitz, Cheaptickets, etc.
The web site with the fewest demons I've found has been Southwest. They've got the most up front, easy to use web site ever.