These color photos of Europe from 1906 absolutely fascinate me (this is from Austria).
Also, after last week's solar storm, I was hoping to see some sweet Northern Lights pics and I was not disappointed. These are awesome.
I've always wanted to see the aurora borealis in person but it's never really worked out (though, to be fair, I've never tried all that hard). The pictures alone look awe inspiring, I'd can't imagine witnessing them live would be like. Whenever I see pics of the Northern Lights, I can't help but imagine what seeing these colors would've meant to people 100 (or 1,000 or 10,000) years ago.
NASA's Astronomy Picture of the Day is always worth a visit. Great pics plus good explanations of what the hell you're looking at.