For those interested in Church History and theology, the internet is a treasure trove. Here are a number of sites you will want to check out.
A wider range of literature is available at the Post Reformation Digital Library and the Christian Classics Ethereal Library at Calvin College and the Theological Commons at Princeton Theological Seminary.
If you are looking for more general classic books, a good resource is The Online Books Page. Those interested in theological works in Dutch should visit Oudvaders.nl. And for those looking to buy rare, used books, have a look at AbeBooks, which gives you access to thousands of bookstores at once.
Of course, this is only scratching the surface–there are so many other sites offering so many other good books–more than you can ever read in a lifetime. And of course the web gives you access to poor quality books with equal ease, but that is inevitable.
Martin Luther once said that other than God’s salvation to mankind, the printing press is probably the best gift that God has ever given to man1. Imagine how effusive he would have been about the libraries mentioned above! Like the printing press, the internet as a technology is not inherently evil but has an incredible potential for good!