Let's start by making one thing clear: there are two types of free museums in Paris, those that are completely free, which do not charge a penny to enter, and those that have permanent exhibitions that are free to enter but with temporary exhibitions that do cost a fee. In this list there are both categories.
Tastes run the gamut, so we'll leave the full list of free museums in Paris, but we'll concentrate more on our favourites, the ones we recommend the most.