The entirety of Alfred Russel Wallace's works can now be found online for free, including papers, newspaper articles, and manuscripts. While I like my paper copy of The Malay Archipelago, it's always gratifying to know that old and often-expensive works can be read without charge - and of course, I'm glad to see that Wallace is getting the attention he deserves.
Wallace was co-discoverer of the theory of evolution by natural selection. Wallace sent a letter to Darwin from Ternate, in what was then the Dutch East Indies, outlining his views on natural selection. The letter spurred Darwin to action, resulting in a paper authored by both Darwin and Wallace and the publication of On the Origins of Species [...] in 1859. It is clear from Darwin's works that he had thought more about evolution and its mechanisms than Wallace had, and Wallace didn't publish very much on the topic, so credit should rest primarily with Darwin. But Wallace was a clever chap and a great naturalist. Later in his life, Wallace became a spiritualist and attended seances - a fashionable thing to do at the time (Arthur Conan-Doyle and the explorer Percy Harrison Fawcett, among many others, became spiritualists as well). This supernaturalist tendency conflicted with his scientific sensibilities, and it is notable that Wallace refused to accept the possibility of human evolution. He believed, as do many creationists, that humans are too intricate and special to have come from an ape-like ancestor. He has subsequently been proven incorrect, of course, and Darwin's position on the subject of human origins has been vindicated (even down to the continent, Africa, on which the bulk of the process of human evolution occurred).
The Malay Archipelago is worth reading if you have any interest whatsoever in Indonesia, Singapore, Malaysia, etc, or if you love animals (the parts about orangutans may shock you a bit, however). The original illustrations are great as well.