Paleo-Future - Our past's dreams of the future.
Retro-Futurismus - Another retro-future site with nice artwork
Osburne World of the Future Books - Usborne Book of the Future & Usborne World of the Unknown: UFOs - I remember reading these in elementary school and still like the artwork
WW1 Solders Blog - Letters from a WW1 soldier being posted to a blog 90 years after they were written
Shorpy - 100 Yr Old Photo Blog (History in HD)
Crime Library - Covers major crimes, criminals, trials, forensics and criminal profiling by prominent writers
Strange maps - Lots of different maps of different styles
Pictures Of The Past (Now in COLOR!) - WWI, WWII, US during WWII & Vietnam
I'll add more as I find stuff I want to go back and read.