<a href="http://www.searchbyinseam.com" title="search by inseam">Search by Inseam</a> is so utterly brilliant, I wish I'd thought of it.  Choose which style of pant you're looking for, then select the inseam length.  From Petite 21" inseams, all the way up to 38" inseams, there is something for everyone.  They even have listings for maternity wear.  You can search by brand and narrow your results according to price.

Pure genius.