From where to where?
Best airlines in the states for 1 way airfaire are probably
Southwest Airlines or
Airtran Airlines. Some of the other startups aren't bad for airfaire as well. Otherwise, you end up getting a cheaper faire if you book a round-trip, and never use the 2nd half.
For instance (I looked this up a cpl years ago):
Round trip faire, delta, savannah, ga to Atlanta, ga; 21 day advance purchase - ~$80-100
One way faire, delta, savannah, ga to Atlanta, ga; 21 day advance purchase - ~$400
Yes... $300 more to take 1/2 the flight... I don't know what they were smoking.
Anyway, I know airtran does all it's faires by 1 way travel, and swa just has decent prices.