British History Online:
British History Online provides access to a wealth of primary sources related to medieval and early modern history, including millions of letters. The Calendar of State Papers are especially illuminating on countless issues. Most of these sources relate to England, but they also provide access to translated copies of letters and sources from the archives of Italy, Germany, France, east and west Indies, etc., in the Calendar of State Papers: France, Calendar of State Papers: Venice, etc. These are incredibly useful sources. In fact, historians have made great use of these sources in their research. The calendars also include information on the British colonies.
Gallica Online:
Gallica.bnf.fr is the official digital repository for the National Library of France. It is an indispensable resource. It will automatically translate to English based on your IP address. The vast majority of the sources are in French, but it has a tremendous amount of sources in English, and you can filter sources by language to choose English on the left ribbon after searching.
Early English Books Online-TCP:
The Early English Books Online (EEBO) database is a massive database of digitized books from British libraries before the eighteenth century. The Text Creation Partnership (TCP) is a group of university to collaborated with EEBO to make text reproductions of a large portion of the EEBO database available for free. At this time of writing, EEBO-TCP had created text reproductions of over 80,000 books. They are available in Phase I and Phase II. The two phases have different books in them, so you will have to search both. This is made easy through the basic-search feature. This is an incredible resource for early modern English-language print.
Eighteenth-Century Collections Online–TCP:
The Eighteenth-Century Collections Online (ECCO) database is a similar database as EEBO but dedicated to the eighteenth century. The Text Creation Partnership (TCP) similarly began a project creating text reproductions of this very large database (over 205,000 books), but they did not make the same progress as with EEBO. So far, only about 3,000 books were reproduced. But it is still a good resource for 18th century English language print from both England and the Americas. ECCO is already text searchable, so the goal for ECCO-TCP was to improve the OCR in those texts reproduced.
Evans Early American Imprints–TCP:
Evans-TCP is a text reproduction of the 5000 the most frequently studied books from the Evans Early American Imprints bibliography (which includes over 36,000 books). It followed the same pattern as EEBO-TCP and ECCO-TCP. This database is similar to ECCO in that it produces more reliable OCR’ed text for the reproduced texts because Evans already makes their digitized documents text searchable.
Archive.org:
Archive.org has a substantial online library of books, perhaps one of the largest. Many of the books printed before the 20th century are available for free since they are outside of copyright.
Google Books:
Google Books has a large library of books like archive.org. Many of the books printed before the 20th century are available for free since they are outside of copyright.