Check out Game Maker: http://store.steampowered.com/app/214850/
- Different components sold as DLC (http://store.steampowered.com/dlc/214850/)
- Achievements (http://steamcommunity.com/stats/214850/achievements) for things like "run a game on iOS" and "1000 debugs"
- Created games can be put on the Steam Workshop (http://steamcommunity.com/workshop/browse?appid=214850), which is INCREDIBLE when you think about it!