The de facto standard for delivering the "WOW!" factor online is Adobe's Flash media platform. With over 98% market penetration, no other browser plug-in comes anywhere near it.
When you publish a streaming video or animated presentation in Flash, virtually the entire world can see it without having to worry about the setup on their computer. YouTube uses Flash to deliver video to every corner of the planet. End of Argument.
More than just a video or animation engine, Flash can connect to databases using XML middleware, process user input with programming logic and interact with PHP, ASP or .Net to produce powerful web applications with rich media content.