Mark a star in your diaries for every first Sunday of the month in Paris. On this day, the city's national museums (with the exception of the Grand Palais, but including such behemoths as the Louvre and the Orsay and offbeat treasures like the Musée Albert Kahn) open their doors to the public for absolutely no charge.
As you can imagine, the intiative is enormously popular, and it's wise to start queueing early. All the museums below participate in the free opening scheme, so you can plan endless hours of cultural viewing without spending a single euro. The queues might be long, but that's the price you pay for a free exhibish, eh?