From the site's Introduction
The Complete Work of Charles Darwin Online (or Darwin Online) began in 2002 to assemble in one scholarly website all of Darwin's published writings and unpublished papers. It does not cover his unpublished letters which were already the focus of the Darwin Correspondence Project.Most recently the site put up Darwin's entire private papers online. (h/t New Humanist Blog)
Darwin Online is by far the largest Darwin publication in history. It contains over 43,000 pages of searchable text and 150,000 electronic images. This site contains at least one exemplar of all known Darwin publications, reproduced to the highest scholarly standards, both as searchable text and electronic images of the originals. The works reproduced here were lent by helpful institutions and individuals. Some of the books are worth over £100,000 ($196,800), which means that few libraries can afford to collect all of Darwin's works. The site also provides the largest collection of Darwin's private papers ever published in c. 20,000 items in c. 90,000 images, thanks to the kind permission of Cambridge University Library.