A great resource for finding free audiobooks is Free Listens. This blog reviews audiobooks, rates them, and organizes them into easy-to-browse categories: length, genre, and when the book was written. The reviews are nicely detailed, letting you know about the quality of the recording as well as the quality of the book.
I particularly like having information about the recording quality. I hate downloading something that interests me, only to find it's essentially unlistenable (is that a word? Oh, well. It is now. I so decree!)