Has a plug-in or activeX control been released?
There is a stable web plugin for Blender v2.25 on the old Sumo physics system. You will have to build web games in 2.25 if you want to use it. Internet Explorer, Netscape, and Firefox are supported. There are also versions available for Linux and FreeBSD.
A newer web plugin for v2.42a with newer Bullet physics is in alpha testing stage. It only supports Internet Explorer (though Firefox users can run it through “IE Extensions”). It seems that the new plugin overrides the old one, causing computers with the 2.42 plugin to incorrectly play games made for the 2.25 plugin.
In a few minutes, I will edit this post with a link where you can find more information.
Edit: I have the most complete list of resources I am aware of listed in my article “How do I publish my game?” This link will take you directly to the Web Plugin section.