The world of HTML5 is in an early phase, but the possibilities are vast already. Compatibility for platforms like iOS/Android, facebook, Windows 8, browsers and Steam is available, and the future is looking rather inviting.
The tool used for some of my projects is Construct 2, by Scirra. Its a powerful tool, which I encourage all game developers to check out. It offers tremendously fast prototyping, and is well suited for fully fledged games.
Have a look at the timelapse below, where i create a game in a matter of hours.