In general when a scientific paper is published through one of the major publishing houses, the publishers require a transfer of copyright. The terms transfer almost all rights to the publisher, but most of the agreements include a clause allowing the authors to share the work through their personal websites.
I don’t have the HTML page done yet (will have titles, abstracts, full papers and related links), but I do have the majority of my publications available for download.
I think that if any public funding was used for the research leading a particular paper, then that paper should be freely available to the public. The publishers aren’t going to encourage this though, so authors of these papers should be sharing them as much as legally possible.