Here are some places I visit regularly.
Warping History: Analytical Methods in Historical Cartography.
This is John Hessler's blog, "Dedicated to the use of mathematical modeling and fieldwork in the study of the history of cartography".
Lots of overlap with my own interests, from historical cartography to differential geometry, sixteenth century science, etc.
Envisioning the World: The First Printed Maps, 1472-1700.
A beautiful site on a traveling exhibition on early printed European maps.
Dead Reckonings: Lost art in the Mathematical Sciences.
A beautiful and inspiring blog by Ron Doerfler on nomography.
Robert van Gent. History of Astronomy.
Excellent repository of information on the history of astronomy.
A very useful site with lots of ancient and medieval scientific texts in the best editions.
The blog of Philip Ball, on "the interface of science and culture".
The blog of mathematician Timothy Gowers.
Terence Tao's blog.
Most of the stuff here is way above my expertise, but from time to time I can follow in detail some discussion.
No explanations needed. The site of Edward Tufte: just go and see.
Bonecos de bolso.
Beautiful drawings and sketches by Pedro Cabral.