You can't get a sensible one-size-fits-all number. It depends on both the content and the plugin. When NBC requires people to download Microsoft Silverlight t

You can't get a sensible one-size-fits-all number. It depends on both the content and the plugin. When NBC requires people to download Microsoft Silverlight to watch the Olympics, they can probably get a very high conversion rate. If you're the IRS or a major employer and you want people to download an Adobe plugin in order to view tax info, then most will probably do it. If you're a company I've never heard of asking me to download a plugin I've never heard of, then I'm going to do it only if you really convince me that I absolutely need to try your service.