British History Online is a digital resource containing some of the core printed primary sources for the medieval and modern history of Britain. Most of the material is free, but some is not. See this link for materials that are NOT free. (Some are available in the database State Papers Online.)
Catalog for the National Archives - Discovery holds more than 32 million descriptions of records held by The National Archives and more than 2,500 archives across the country. Over 9 million records are available for download. Searching is free but there may be a fee to download records.
The Avalon Project from Yale Law School includes digital documents relevant to law, history, government, etc. from ancient times to the present.
Georgian Papers Online: This catalogue currently contains descriptions and digitised images of over 200,000 pages of documents dating from the reigns of George I to William IV, including personal letters, diaries, account books and records of the Royal Household.
