bstetric Ultrasound - A Comprehensive Guide - A great site with the what, when, why, and how, of ultrasounds during pregnancy. Some of the information is pretty technical, but the pictures are terrific. They're even labelled, so you can tell if you are looking at an ear or a foot :)

Ultrasound in Pregnancy Web Book - Dr. Applebaum - A comprehensive, easy to read book online. Dr. Applebaum created it to answer the many questions that parents-to-be have about ultrasound. It is less technical than the site listed above, but will most likely answer any question you might have. Even talks about the full bladder issue:) Excellent!