Construct 2 is a HTML5 2D game engine that allows non programmers like myself to create games. It uses a Drag and Drop visual editor, a Behaviors system and easy to use event system that does all the programming for you.
Place a sprite representing a floor and give it a solid behavior and another sprite above it representing the player and give it the platform behavior and voila! You've created a platormer. Can tweak the platform behavior as well, like gravity, speed, jump strength, etc....
The event system is simple to use as well. We could add an event that states Player >On left arrow is pressed = Player >Set Mirrored in order for the player sprite to face the right direction or Player > Platform On Jump = Player >Set animation to "Jump" its as simple as that.
Once your done your game, you can export your game as HTML5 and publish it to your or many other websites. If you've installed (and I recommend you do) NodeWebkit, you can export your project as a Windows, Linux and Mac application. Scirra says build once publish everywhere, sometimes with a little work do. If you want to publish to Android or IOS for example Construct will export the files needed in order for you to use Cordova or Intel XDK to build your app.