I thank correspondents off-list who drew my attention to Scirus, to Google
Scholar, and to the ISI Web of Knowledge (pay to use; not yet tried).
On list, Les Carr wrote "While OA stands at 15% or less, there is very
little point in putting a citation-following interface to just the OA parts
of the literature ..."[1].
I hope this does not mean that nobody is working on development of
free-as-in-beer version of Scopus.
Seems to me that capable software to get material _out_ of repositories
should now be even higher priority than ditto to put it in.
How else will we arrive at "a seamless, completely interlinked learned
literature at the fingertips of every scholar and scientist in the world"[2] ?