Between amazon.com and audible.com, I spend more than I would like to publicly admit on books each month. I can rationalize to myself that they are investments and not expenses, but the return of most of them (career books aside) is usually intangible at best. With the internet and the blogosphere I can satiate a large amount of my addiction to new information each month for free, but finding a real book (few hundred pages, professionally edited, etc.) for free on the internet has proven quite hard up until now.
But that's all in the process of changing. Two new services are looking to partially surplant Amazon and Audible (at lesat for books in the public domain), and they are doing so for free.
Project Gutenberg has a library of 17,000 ebooks, most of which are classics such as T.S. Elliot, and Shakespeare which can be downloaded for free. They even have a decent selection of human read and computer read audio books.
LibriVox is a newcomer with an ever expanding library of audiobooks. Their selection is still less extensive than Project Gutenberg's, but their site design and ability to download via podcasts makes them a bit easier to use. They are new and growing and I'm looking at them to eventually dominate the free public domain audiobook space.
These two services have their limitations (mainly that they are limited to public domain books), but 17,000 books at Project Gutenberg is more than enough to provide a small library of entertainment. And you can't beat the price - 100% free.