I was in Paris on Sept. 7. As it was the 1st Sunday of the month we wanted to see some free museums. Started with the Rodin when it opened. Enjoyed the house and garden. Walked over to the D'Orsay which was packed. The line looked at least 2 hrs. long. Skipped it and went to the l'Orangerie to see the huge Monets. Lovely space to view these wonderful paintings. Then after a quick late lunch off to the Louvre. No line to speak of. Very crowded inside to see the Mona Lisa, but other areas not so crowded. I was almost put off by other's comments about free museum day being so crowded, but we enjoyed all we saw. With everything being so expensive, it was a nice treat.

Thanks for posting about your experience. I'll be there for the first Sunday in November...

In 2001, we tried to go to the Louvre on the first Sunday and the line was just too long. It was maybe around noon or so. In June 2007 we went around 4pm and there were no lines and not that many people inside. It was quite enjoyable. Like you said, we had been warned and expected the worst, but that was not the case at all.

Try the "Carnavalat" in the Marais. We've been there on two different trips on different days, and both were free. Plus it is one of our favorite museums in Paris!

I was just in Paris for the first Sunday in October. I'd planned on hitting as many museums as possible, but ended up with just the Louvre and Orangerie.
Got to the Louvre when they opened at 9AM and it was fine. By the time we left, it was packed.
At the Orangerie we had to wait about 20 min to get in as they strictly control the numbers inside.
By the time we got to the D'Orsay, like you, the line looked to be a couple hours long so we skipped it.

I went to the Quai Branly on the first Sunday this October and, at ll:00, there was no line. Later in the day, I went to the Cluny, around 4, and there was about a 10-minute wait.

Wednesday and Friday evenings the Louvre is open late and, if you go after 6pm, it's both cheaper and much less crowded. Wednesday is better than Friday.