One thing that seems to be missing from all of the audiobook apps in the Marketplace is the ability to easily bookmark your spot in an MP3 and return to it.
XDA member, dbroome, has developed AudioBookMark, which as the name implies, does just that. AudioBookMark not only remembers which MP3 file you are listening to, but where in that file as well. And in case you are someone who likes to skip from title to title, you can select from a list of your books and dive right in where you left off on each. You can use the built-in transport keys to speed to a certain point of an MP3, or manually enter HH:MM:SS to jump directly to the juiciest parts.
You can download it here for $.99.