Last Saturday night was the Nuit Blanche festival in Paris- The "White Night".
Museums, galleries, churches, and institutes all stay open all night, with the metro running all night to accommodate. There's lots of concerts, both official and unofficial, and thousands of people come out and stay out all night for the occasion.
While some of the lines were really long- who wants to go to the Pompidou at 2am? lots of people, apparently- some of the smaller happenings were nice, if not a little weird.