The Royal Society's digital collections contain printed works, manuscripts, pictures, objects, video recordings and other material reaching back to the foundation of the Society in 1660. It also provides access to the Philosophical Transactions, which have been fully digitised from the first issue in 1665.
The British Library's many catalogues provide an easy way to locate almost any text published in the British Isles, as well as the BL's extensive holdings of archives and manuscripts.
HathiTrust Digital Library provides full-text access to thousands of books, particularly those published before 1850.
Wikisource provides full-text access to the entirety of the Dictionary of National Biography's first edition in an easily searchable format.