Jureeka! is a browser extension for Firefox and Chrome built by Michael Poulshock now in collaboration with Cornell’s Legal Information Institute that turns legal citations into hyperlinks to open access versions of the material cited.
Somewhat surprisingly, it works with some citations to British materials and to Canadian materials, principally federal statutes and Supreme Court of Canada cases on CanLII, so far as I can judge. This is no substitute for the high-end commercial products and neither for CanLII material. But if you find yourself working with US material in HTML outside either of the biggies, you might find this plugin useful.
It would help if there were a full statement of which citations, US domestic and foreign, it captures — there is a list in the sidebar of the Jureeka! blog that lists some of the US citation forms the plugin captures. It would also help, of course, if something akin to the now commercial LexCite from Lexum were available free for documents referencing Canadian legal materials.