The Stencyl game creation tool which has been in development for a couple of years has finally released 3 games which are fully playable within a browser that have been created using its engine. The project, which no longer plans to be open-source, had been labelled vapourware by many as little had emerged publicly from the team behind the project.
All of the released games are platformers.
A release date has still not been announced but there there is a beta program which you can apply to join – if you try and get any joy please let me know.
Self-promotion disclaimer/excuse for a link: I have a terribly out of date blog about Stencyl (formerly Stencyl.org).