Just to chime in, I believe that HTML5 currently is as "cross-platform" as it can get, especially for games. Two examples made with Impact:
- Olympia Rising[1] - this game recently had a successful kickstarter campaign and will be released for various desktop platforms and maybe more later.
- XType Plus[2] - my own game coming to the Nintendo Wii U, using the Nintendo Web Framework. It's a pimped up version of this XType[3], which you can play in your browser right now.
[1] https://www.kickstarter.com/projects/paleozoic/olympia-risin...
[2] http://www.nintendolife.com/news/2014/02/developer_interview...