Sandbox games are the easiest genre to try in a browser because most of them start immediately. Minecraft Classic is Mojang's own free browser build, Eaglercraft is a community client, and Craft is a small open-source voxel game with free assets. The Grand Theft Auto entries are a different story: Vice City expects a game archive you build yourself, and the GTA 5 link in the original list only survives as a web archive.