You can open a web browser window inside the 3D window (note: if you set the size of that window to 100% * 100% with both top and left set to zero, the 3D is invisible to the user, but is still there). It works just like a normal browser window, you can click links or fill forms, etc.
Examples of use:
-- an online shop that gives you the secure form to order the object you have been clicking, or its details & availability
-- displaying a Flash or Shockwave animated & interactive advertisement banner
-- a poll system that displays the results in both 3D and html
-- for 3D communities, a window displaying a chat applet or instant messages
-- an eye-candy 3D web browser
-- opening a page with an activeX application embedded
-- etc, the applications are endless... | 
