Speedriding in Chamonix

In Chamonix there's a few places where you can go speedflying (I know you're asking about speedriding but I hope my comment on speedflying would be helpful too). I did my first flights from Brevent this winter by myself, the landing field is just next to the car park down by the Planpraz lift station and is pretty big. Another option is Grand Montets with a landing by the road (details can be found here: http://www.chamonix.com/pdf/vol_libre_en.pdf). If the wind up at 3800m (Aiguille du Midi) allows and you feel brave you could try to fly off the north face into the valley, although I highly recommend a guide or a fellow pilot who knows the area for this, there's not much room for error up there. See speedriding/flying videos with the names that I've mentioned online to get an idea what it's like.
But before you even seriously consider flying in France make sure you have an appropriate 3rd party insurance (the only legal requirement). And read the Vol Libre booklet, I can't stress enough how helpful it is.
Fly safe, have fun
P.S. You can also find a some info about speed-riding/flying in Chamonix on pgforum.