One remarkable thing in Paris is the quantity and quality of street art. I don't mean crappy tag vandalism, but thoughtful and beautiful art by folks like Jèrôme Mesnager, L'Atlas and Space Invader. Walking around Paris is fun for all the beautiful old buildings, but look closer at some of the new stuff illicitly stuck to the walls, too.
Flickr is a good place to find images of Parisian street art. The Paris street art group is good, as are the photos of yoyo (and his website), tofz4u, chais, vitostreet and others. But I'm really enjoying stumbling into them myself. Many of the installations are quite big and ambitious; the nights must be quiet.
I've been taking photos of street art when I can. I'm particularly taken by the Kufic stylings of L'Atlas; Arabic calligraphy is beautiful, and somewhat provocative in Paris. And I recently stumbled into a beautiful gallery at 23-25 rue Ramponeau in the 20th. There's an artist workspace nearby, so I imagine this was semi-authorized.