The excellent and ever expanding Luminous Lint has a fascinating online exhibition of 19th century Memento Mori photographs. This relatively brief fashion (?) is often said to be a response to the new technology being made available to the masses, which doesn’t explain the majority of examples being shot in the USA.
The second part of the exhibition is dedicated to photographs of violent or abrupt death, if that’s your cup of tea.
I had a quick look, it’s not mine.
Other notable online exhibitions and archives
Terence Nunn’s photos of disappearing London
The Lesbian and Gay Newsmedia Archive regularly puts small collections online.
The San Francisco based GLBT flickr group
Life magazine’s astonishing online archive
Large collection of Haight-Ashbury from the ’60s to the present photos
[See also this post]