Windows 8.1 / Windows 8 / Windows 7 / Windows Vista
Garry's Mod is hands down one of the best sandbox indie games of all time.
Garry's Mod, also known as “GMod”, is an amazing indie game that lets you create any kind of games you like and share them with other users – just keep in mind that the latest version is paid but you can always download and share mods for the game for free from the Steam Workshop.
The Steam Workshop is really what sells this game, honestly. If you’re creating a game (or even a feature film!) and you need assets, you can just go check the Workshop and see if there are models, textures, or scripts that someone else already made so you can get the work done.
There are literally millions of free maps, mods, add-ons, and even free full playable games made in Garry’s Mod that you can enjoy. They have RPGs where you can be a hero shaped like a butt, Mario-Kart clones, you can fight terrorists in Baghdad, you can do anything with the Mod and what you can’t do you can make for others to enjoy.
In short, if you’re looking for a framework that lets you explore and share cool ideas, GMod is definitely for you. You’ll have to buy it but after that you’ve got unlimited access to millions of cool things generated by a community that’s extremely devoted to The Mod.