Audiobook Builder is one of the apps that I use about six times a year, but it’s easily worth the $4.99 purchase price. It allows you to combine audio file, create chapter stops, and then end up with a .m4b audiobook file. This is especially helpful if you have a collection of lectures or seminars that you want to listen to. With an easy drag and drop interface, you can take dozens of audio files and send them to iTunes as a single file. Since it uses the .m4b extension, your iPhone will recognize it as an audiobook and remember playback position. You can also use variable playback speed as well.
It can import files from iTunes, the Finder, or even a CD. The CD option is helpful if you have some Audiobooks on CD and want to convert them into digital format. Depending on the book, you can often find the CD cheaper than the Audible version, so this app would make it a breeze to get it converted.
Get it on the Mac App Store for $4.99.