The complete works of Charles Darwin is available online at: http://darwin-online.org.uk/
Apparently, handwritten letters were recently added to the Darwin Correspondence Project at: http://darwin1.caret.cam.ac.uk/
It's a pretty big collection:
Darwin was a prolific letter writer, exchanging correspondence with nearly 2,000 people during his lifetime (1809-1882). Nearly 14,500 of his letters are known to exist, with the biggest collection residing in Cambridge.Makes me wonder how Darwin would have felt if he knew that so many of his personal letters would someday be publicly available to anyone with an internet connection. Seems an ironic devolution of privacy.
The article below mentions the variety of his letters, including some family conversations about his personal hygiene. I wonder if that's as embarrassing as baby bath pictures?