I'm sure many tourists know about this already judging from their numbers on this particular day... Every first sunday of the month is a Free Museum day in Paris... I dont know if it applies outside of Paris, maybe it does. I can't find a list of museums that participate to this but most of them do. The Louvre and the Orsay for sure. The queue will be really long especially for the 2 mentioned. As for the Louvre, go early, but there's no need to go TOO EARLY, 9-10am is alright, the queue will be long but it will move quite fast. I guess one of the best place to queue would be the Carousel du Louvre, which is linked to the metro station. The popular places in the museum will be stuffed with tourists, who seem to be interested in a shot of the mona lisa and venus, but if you're there for the art like I am (ahem!) the rest of the museum should be quite clear.