Scirus, the free scientific information search engine provided by Elsevier, includes millions of sci-tech documents, including data for some 21 million patents obtained from LexisNexis. Here’s an exact breakdown of Scirus’ patent coverage:

EPO from 1978 (grants and pregrants)
JPO from 1976 (pregrants, English abstracts only)
UK from 1916 (pregrants)
USPTO from 1790 (grants#) and 2001 (pregrants*)
WIPO from 1978 (pregrants**)

# US coverage does not appear to include plant patents before 1976.
* The USPTO began publishing utility and plant patent applications in 2001.
** Published PCT applications.

Many thanks to Danianne Mizzy of the Engineering Library, University of Pennsylvania for this scoop.