What is the cheapest airlines to fly from washington dc to chicago every weekend?

Assuming you want to fly from O'Hare Regan it is American or United (don't use Dulles or BWI if you are flying to the District, those are okay if you are going to VA or MD). Both have several non-stops everyday. Book at least a week or two in advance and you'll only pay about $150 RT. I prefer American because there is slightly more legroom, the United flights have two sections of economy "Economy Plus" and "Economy" - the regular economy (the cheap seats) is a bit more cramped. If you prefer Midway, ATA has cheap flights to Regan.

Southwest

try to look at www.cheapestfare.com

If you don't mind flying out of Baltimore, you can catch a flight on Southwest from Baltimore to Chicago's Midway Airport. Go to Southwest Airline's website. www.southwest.com Because Southwest operates out of there, I am sure United and American have also lowered their prices out of that city to Chicago O'Hare to be more competitive.