Project Gutenberg is the brainchild of Michael Hart, who in 1971 decided that it would be a really good idea if lots of famous and important texts were freely available to everyone in the world. Since then, he has been joined by hundreds of volunteers who share his vision.
In essence books which have no copyright are available for download as text files. This includes old works (I get my Wodehouse, Jerome K Jerome here), technical works which never had copyright on them etc. These are all perfectly legal and can be downloaded directly from the websites.
This is only one of many servers throughout the world. Just do a Google search and you will find many more. Anyone interested in e-books should have a look. You may even wish to join in and help the project.