I have just put the first alpha version of RPG Playground online at http://rpgplayground.com. Remark that this is an alpha version, so nothing works, except for the most basic functionality. You can walk around with your game character, and when pressing the ‘edit’ button in the top-left corner, you can add and remove objects/tiles. So it’s already possible to create some cool looking levels.
Unfortunately saving levels isn’t implemented yet, so once you close your browser, your newly created level is gone. But don’t worry, it’s the next thing I’m working on.
So go ahead, experiment with some level creation at http://rpgplayground.com, and let me know what you think, or send me some screenshots of your work.