Musee D'Orsay is directly across the Seine from the Louvre, but it is another world entirely. The gallery is housed in a hugely impressive building that was originally a railway station. It houses artworks, sculptures and some furnishings, all from the period 1848 to 1914.
The gallery has a fantastic collection of artworks by Monet, Manet, Renoir, Cezanne, Degas, Van Gogh, Toulouse-Lautrec, and many other artists of the period. These are displayed over three bright and spacious floors, with none of the crazy crowds and tour parties you find in the Louvre. There is also a large collection of scuptures, including many by Rodin, and the original scale model of La Libertie (the Statue of Liberty).
There are 3 cafes inside, we had coffee and cake at the one on the ground floor. The queue initially looked long, but it moved quickly and the quality was good.
Entrance to the permanent collection is €9, another €3 will get you into the special exhibitions too. We spent about 4 hours happily in the permanent collections and could have stayed longer, so only pay the extra if you really want to see the exhibitions. As far as I could see, the gallery is wheelchair friendly and we saw wheelchairs inside.
One word of caution! We walked there from our apartment near Notre Dame, about a mile each way. We were stopped twice on the way there and once on the way back by scammers. There is a scam widespread at the moment where someone appears to 'find' a gold coloured wedding ring on the ground in front of you. I'd read about this, so we ignored them, but i think the aim is to either extract money from you for the ring, or to empty your pockets while you are distracted by the scammer. Beware!