JavaScript gives you the most possibilities because you can place semi-transparent items anywhere in a webpage, they can resize with user interaction and move down a webpage as a user scrolls. JavaScript can also be used to re-write a webpage, including changing images, fetching more text, revealing optional form elements and the changing the destination of hyperlinks.

Some, if not all, of this can be achieved with IFrames and Flash but web browser security is such that you'd have to include JavaScript anyhow.