Well, many of the ones I post are just from
my blog archives, though I find many on other art blogs (though sadly the best ones often
end up shutting down) as well, or though aimless googling.
50 Watts is a good one for old obscure book illustrations and the like.
Monster Brains is fantastic all around (
LOOK at these amazing old Thai horror film posters!).
Unquiet Things is great if you like strange and macabre type stuff in general.
And there's some wonderful general art/photography tumblr blogs that have been around for several years like
Nevver,
2 Headed Snake and
Barns Burnt Down Now.
.. For photography specifically I like
this one and [nevermind, they shut down too]
Some online magazines/other blogs:
This Is Colossal,
BeinArt,
Strange Attractor