Biodiversity Heritage Library: http://biodiversitylibrary.org/About.aspx
BHL is an ambitious project to digitise the World's biodiversity literature. The (mostly American) partner institutions include our own Kew Gardens and the Natural History Museum. The digitised literature is placed online for public access and the material is maintained using the best practices of digital preservation.
When using BHL, be aware that the dowloadable PDF's are sometimes "optimised" to such an extent that any drawings or plates are reduced to meaningless smudges. In such cases it's best to download the "zip" file and then build your own PDF's, or at least substitute the plates from the higher quality versions.
BHL are always looking for publications to add to the archive. If you're on the editorial board of a learned journal, or even a member of a society which produces a journal, this could be the way to make your back issues more widely available. In my experience, the scanning is carried out at no cost to the society and the society is given a copy of the resulting files.