My 2 cents, based on my experiences:
Southwest has been the cheapest way to fly 100% of the time. The only times I don't go SW is when I am going to a city they don't serve. (I check out prices every trip though, just to see if again they are the lowest... so far, they are!)
Travelocity can get you a good deal on the regular airlines,
BUT A CAUTION: I used them in 2000 to book a flight from Albany, NY to Oakland, CA (I forget the airline I ended up on, may have been American.) Anyway, I thought it was odd that a week passed with no email confirmation sent by the airline itself -- only from Travelocity. So I called the airline - and they had NO RECORD of the flight being booked! Luckily, my trip was still months away, and I got it worked out with the airline in time, and at the Travelocity price. So, buyer beware, make sure those tickets get put through to make that "great deal" worthwhile!
