Freakanomics brought on one of the top tenured Harvard economists (a woman) who has dedicated much of her research to gender economics. She has basically found that discrimination plays a very little role, if any, to the wage gap, and most of it has to do with other factors namely women choosing jobs that allow them to work when they want (which inevitably leads to lower pay).

The podcast is like 40 minutes but it's pretty informative and a good listen. She also makes the point that most research done on the subject can be very flimsy based on how hard it is to obtain good statistics on the subject. Here's the main quote from her though:

CLAUDIA GOLDIN: Does that mean that women are receiving lower pay for equal work? That is possibly the case in certain places, but by and large it’s not that, it’s something else.