GameMaker
GameMaker is the engine used to create all of Pirate Software's video games (with the exception of the Heartbound ARG, which is developed in HTML). It was originally created by Mark Overmars, then development was taken over by YoYo Games, and finally by Opera.
The current iteration of GameMaker is subscription based, at $5/month for desktop development and $10/month for desktop + mobile/web, but Thor has a lifetime license, since he bought the engine before Opera took over its development and changed the pricing model to the current one.
GameMaker also allows free exports to Opera's own game store, gx.games. However, the store has very limited popularity and discoverability and is not suitable for larger projects as it does not allow the game's progress to be saved.