A very basic but extremely easy to use database of 500 diarists, from Charles Abbot to Walter Yonge, from Jikaku Daishi in Japan 838-848 ad to Deborah Bull in 1999. There's a page on the data for each, including a bio, and what's in print, where the originals are, and internet links.
I don't know about anyone else, but for me there's no substitute for diaries and letters, for researching voice and manners and mores as much as fact.
http://www.pikle.demon.co.uk/diaryjunction.html
Thanks to Grumpy Old Bookman for the link.
Emma