What's wrong with using Kayak? I have been searching for some last-minute fares recently and American have come up cheapest. Generally, Virgin America IAD-SFO is cheap. Also, in general BWI and OAK are cheaper due to the Southwest effect.

But it's hard to generalize too much -- I fly this route at least once per month on average, usually with a red-eye return, and the results run the gamut. I try to take United whenever possible, and usually IAD-SFO, IAD-LAX-SFO, or BWI-LAX-SFO are competitive. But not this month!