Southwest flies out of both BWI and IAD, but they do not fly nonstop to any Los Angeles area airports from the DC area, and they do not serve Long Beach airport at all. Thus, Southwest may not be the best choice for your particular route.

Kayak and Sidestep are very similar...and the two companies have actually recently merged. I find both websites useful. Neither one, as far as I can tell, directly prices Southwest flights, though. You'd have to go to southwest.com to check Southwest's fares against all the other carriers, which are listed on kayak or sidestep.