I got a chance to check out My Game Builder, a Rich Internet Application that allows developers to create their own game worlds. It has a powerful building engine that lets you touch every conceivable part of the game and customize it down to the smallest detail. At first, the interface is a little overwhelming, but the team has a tutorial meant to walk you through the steps to creating a game, importing graphics, and getting started. As you move through the tutorial, more advanced controls open up so you have even greater control over the game world. They also have a video up on YouTube that shows you how to build a game in five minutes.
When you're done creating your game you can switch over to the game player which will let you test it out and see what you've created. There is also a publish button in the Map Maker so that when you're done, you can share the game and have it show up on a list with other games. To test out some games, you can go to "Browse Other Users' stuff" on the main site and see a list of users and then the game(s) that those users have created. Soon it's going to list it by game so that you can see what's been created instead of digging through users.
My Game Builder was built using Flex technology and is using Amazon's S3/EC2 services to host. They're also using Ruby on Rails on the backend. It is a very cool use of Flex and Rich Internet Applications in general. When I was a kid I always wanted to create video games and things like RPG Maker were a lot of fun. My Game Builder is in that mold and requires nothing more than a browser and the Flash Player. It's aimed at capturing the fun of those tools while providing a bit of a social network to share assets and ideas. Right now this is a preview release, so the UI is still being tweaked. Down the road the site is planning on allowing users to embed games on their blogs or social networks pages so that users of the system have a way to virally share content.