New on the Ontario Genealogical Society website, a unified search capability across many of the library catalogues of OGS Branches. Find it under the Libraries tab on the top banner.
Included are holdings of the OGS Provincial Library at the Toronto Reference Library, the British Home Child SIG, and various branches — Essex, Halton-Peel, Huron, Kawartha, Kent, Niagara Peninsula, Ottawa, Oxford, Quinte, Simcoe, Sudbury and Waterloo.
The detail available varies. Ottawa has more comprehensive descriptions of the branch library holdings than the other repositories.
A quick rundown of other library catalogue sites of interest.
Worldcat: 2 billion items
Aurora: LAC Collection
Voilà: Canadian National Union Catalogue via LAC
The British Library
US Library of Congress
FamilySearch Digital Library
Internet Archive Books
Google Books
HathiTrust Digital Library
and local public and university libraries.