Scopus has a new site listing the most cited papers (published between 2004 and 2008) in various scientific fields. In the overall top 20 list of most cited papers, only #10 (Journal of Molecular Biology), #15 (International Journal of Computer Vision) and #18 (Lancet) are not open access for anyone to read at the publisher's site, and two of those are available online as preprints.

See also:

Citation Advantage of Open Access Articles. Gunther Eysenbach, PLoS Biology. 2006; 4(5) e157
and the discussion around it.