Pignonfixe.com
Constantly updated. Rides, alleycats and polo meetups. Helps to know the slang. There is an alleycat on March 7th.

Paris Fixed blog
Most of these guys speak some remedial english and you can try to email Yann and find out where the rides are. Ths big avenues and roundabouts are fun but scooters are a force to be reckoned with. You'll find less cobblestone to the south of the 5th & 6th. The north is hilly. Less traffic in the posh 16th & 17th. The central artery that goes along the Seine is closed to cars on Sundays for a few hours and is open to bicycles, skaters and runners only. Nice scenery. I love that city more than anything else in the universe.

Cyclope The premier bike shop.
Paris Brakeless