Looking for OA copies of needed articles

Sarah Washford, Free Journal Articles, Info Junkie, January 22, 2007.  Excerpt:

...When we receive an article request in our interlibrary loan department we tend to send it straight to the British Library and usually a quality scan appears in my inbox within 24 hours.... Today I received a list of article requests from a reader that were all from medical journals and decided to investigate whether any could be obtained free online - I found 4 of the 8 and thus saved us approx £20. I also saved the reader £6.00 in request charges which I'm sure she appreciated....I can't believe that I'm the only one who doesn't usually have time to hunt down journal websites in case there may be a free copy of an article out there somewhere....

[H]ow many students and public library authorities are paying for articles that they could get for free? How easy is it to find free articles? Is PubMedCentral a unique site or are there similar sites out there for other subjects? ...