The Sherpa-Romeo website has recently been updated to highlight the growing number of publishers that allow authors to deposit the publishers' version or publishers' PDF of a journal article into the author's institutional repository. There are now 50 that do so, including such important publishers as Duke U. Press and California U. Press. As Jane Smith points out via the JISC-Repositories list: these 50 publishers "allow immediate, un-embargoed deposit into repositories -- even more allow use in restricted circumstances. This means that there is a large volume of work which can be deposited directly into repositories even if the author has not retained their own final draft. We hope that this information will help repository administrators in encouraging deposit into their repositories."
For a list of these publishers allowing authors to deposit the publisher version or PDF of their article in an Institutional Repository, without fee or an embargo see the following: http://www.sherpa.ac.uk/romeo/PDFandIR.html