Audible.com
Audio books have been available online for quite sometime, but have not made it to the mainstream. I have been an Audible subscriber for over three years. You can purchase individual titles (there are thousands to choose from) and they offer subscription based plans. For a monthly $12.95 fee you are entitled to download two titles each month, regardless of the actual price of the title. This is an extremely good value since some titles are priced in excess of $16 if purchased separately. Playback options include burning the titles to CD-R (for listening on your home stereo and in your automobile) and support for a variety of hardware platforms including PC, Pocket-PC, and several mainstream MP3 players.