Try this one - they check all the travel sites on the web and present the flights in order of price:
www.sidestep.com You'll need to download an add-on for your browser to use it.
I just checked a hypothetical round trip from Oct 1 to 11 from Phoenix to Washington DC and it found 45 flights from 15 sites that ranged from $167 to $471 round trip.
Be sure to check flights into all 3 area airports - Baltimore, Reagan National, and Dulles. You may find a bargain at other than Baltimore that could be worth a bit of extra driving.
[This message has been edited by JChidester (edited 07-14-2001).]
[This message has been edited by JChidester (edited 07-14-2001).]
[This message has been edited by JChidester (edited 07-14-2001).]