If you’re familiar with the idea of Little Free Libraries of books, a Free Little Art Gallery is basically the same concept but for small artworks.
Anyone can play! Little kids, professional artists, or adults puttering around with waterpaints, whatever. You don’t have to be an expert to create something someone else will value, or that will make them think.
The first FLAG (so far as I know) was founded by Stacy Milrany of Seattle, Washington, USA. There’s no official organization or formal qualifications–anyone can just set up their own. The map on this site tries to track them, but I know there are many we’re missing, especially those without an online presence.
In addition to the “Take art/leave art” galleries as described by the sign above, there are some display-only galleries. Again, there aren’t official rules anywhere, but I prefer to exclude these from the term FLAG because they’re only free in the sense that you can look, as you can in any human-sized retail art gallery. On this site we use the word free to denote not only that you can take away pieces you like, but also the democratic aspect. There’s no gatekeeper–anyone can contribute their work.