I run it, so this could be spam, but http://www.rsvpair.com/ is exactly what you requested - a free directory that lets people who want to fly charter find operators, give feedback and see prices, both for large executive jets, turboprops, and smaller props like you were requesting here.

This is just another way for them to get you to spend more money. It might be a buck and it might be 10 hours but if I am going to go through all the trouble of downloading an ebook I would like to take as much time as I like to read it.