Here are some places I visit regularly.
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.
A beautiful site on a traveling exhibition on early printed European maps.
A beautiful and inspiring blog by Ron Doerfler on nomography.
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.
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.
Beautiful drawings and sketches by Pedro Cabral.