Datasets

http://openplaques.org is an open source project (both code and data) has data on over two and a half thousand commemorative plaques, and new plaques are being added regularly. Where available, the data for each plaque includes the:

• Inscription
• Person being commemorated
• Geo location
• Address / country
• Organisation that created the plaque (around 250 so far) 
• Colour
• Roles (e.g. author, inventor, actor, prime minister, namer of clouds etc.)
• Verbs (e.g. lived, worked, died, visited, built, founded) that link the person to the location
• Link to creative commons photo of plaque on Flickr

http://www.nationalarchives.gov.uk/ is the UK government's official archive, containing over 1,000 years of history, with detailed guidance to government departments and the public sector on information management and links to other historical archives.

http://www.oldweather.org/crowdsourced transcriptions of worldwide weather observations made by Royal Navy ships around the time of World War I. These transcriptions will contribute to climate model projections and improve a database of weather extremes. Historians will use your work to track past ship movements and the stories of the people on board.

Archives of newspaper and science journals (eg http://www.nature.com/nature/archive/index.html not free unfortunately) are useful for cross-references (@zzgavin)

historical maps are interesting, various geotiles exist (@zzgavin will hunt out urls)

Victorian London and Sherlock Holmes annotation websites are useful too (@zzgavin)

English Heritage monument data is available as ESRI shapefiles. Their use is 'generally unrestricted' as long as it is not 'used for purposes which may lead to damage to archaeological sites, historic buildings and landscapes'. Includes listed buildings, scheduled monuments etc, with a little bit of data (date of scheduling, listing grade etc). http://services.english-heritage.org.uk/NMRDataDownload/ Needs a login, but is easy to obtain (and I have one!) @mdgreaney

I have this data as points for the centre of each NMR, which has also been converted to lat/lon and WOEID, if anyone is interested. Currently using it to work out if detecting is happening in the vicinity of scheduled sites. @portableant

OS 1:50K gazetteer - has antiquities and Roman sites. Also converted to Yahoo WOEID. You can get the original dataset from http://parlvid.mysociety.org:81/os/ and I can send you my enhanced data. @portableant

http://hansard.millbanksystems.com has speeches, questions, written answers and ministerial statements from parliament going back to 1802 - it's about 95% complete. It also has a rudimentary API at http://hansard.millbanksystems.com/api.

Old Bailey Online (http://www.oldbaileyonline.org/) has records of about 200,000 criminal trials conducted at the Old Bailey between 1674 and 1913.

London Lives (http://www.londonlives.org/) has a collection of 3.35 million names associated with 240,000 documents, coming from 1690 to 1800. 

The National Maritime Museum

The National Maritime Museum could make the following datasets available if there's interest (@foe):

Research databases

• Maritime Memorials (approx. 5,000 records, which can be retrieved via a simple API by ID, word/phrase or KML endpoint)
•  Marine Society’s registers for boys sent to the Royal Navy during the Seven Years War. They're interesting as a history of the poor and for genealogists in search of an ancestor who went to sea as a boy (approx. 5,000 records, available as CSV)
• Database of Royal Navy victualling during the French Revolutionary and Napoleonic Wars 1793–1815:  (Sustaining the Empire, over 4,000 records, available as an Access database)
• Warship histories circa 1500 to 1950, detailing captain, where the ships went and the vessels they encountered (approx. 65,000 records, available as CSV)

Transcripts

• Journal of Victor George Hayward, who died laying provisions for Shackleton (word doc and images)
• Flinders' letters (approximately 150 docs and supporting images)
• Caird Catalogue, which describes the museum's founding collection

The National Archives

We are currently working to make available the following data sets. (Note: This is a provisional list and may change). Please do get in touch with me @mentionthewar if there's something you're particularly interested in working with so we can focus our efforts a bit.

• Complete dump of our catalogue (hold tight it's apparently 10.5GB)
• Selected (more manageable) catalogue subsets (medieval petitions, Victorian photographers and others)
• Full OCR'd text of the Cabinet Papers (covering Cabinet Meetings 1916-79)
• Full transcribed text of MH 12, Victorian poor law and workhouse records
• Our growing gazetteer relating historic placenames to modern ones
• Data from the Domesday Book (I need to check what we have available)
• Geodata for Dixon-Scott collection of UK interwar photographs