You can find on this website a collection of free tilesets that can be used in commercial and non-commercial game projects. All the tiles are the property of their respective authors and Has Graphics only features them. All the tilesets are inspected to make sure they meet certain quality standards, and most important that they are available for non-commercial and commercial game projects.
All the tilesets highlighted here are free to use in commercial and non-commercial projects but for most of them you have to credit their credit and/or link to their respective authors; if you use them so I advice you to link them. After all you give something in return to the people who spend their time creating and giving sprites for free.
You will not find on this website tilesets ripped from other games. I know there are tons of such things but they are out of the scope of this website: to publish free tilesets available for commercial and non-commercial games.
Before using any tilesets please check yourself the license to make sure you are allowed to use them in your game.