Southwest is doing a 72 hour promotion that ends tonight at 11:59 pm Pacific Time. 

0-374 mile flight is $25 one way
375-549 mile flight is $50 one way
550-999 mile flight is $75 one way
over 1000 miles is $100 one way

Travel from Dec 2 - Dec 16, 2009 and Jan 5 - Feb 10, 2010----select destinations

 http://www.southwest.com/