The answer is no, of course. And Canvas is not a plugin. That said, here is an interesting proof of concept blog and video from Alexander Larsson: a GTK3 application running in Firefox without any plugin.
GTK is an open source cross-platform GUI framework written in C but with bindings to other languages including Python and C#.
So how does C native code run the browser without a plugin? The answer is that the HTML 5 Canvas element, already widely implemented and coming to Internet Explorer in version 9, has a rich drawing API that goes right down to pixel manipulation if you need it. In Larsson’s example, the native code is actually running on a remote server. His code receives the latest image of the application from the server and transmits mouse and keyboard operations back, creating the illusion that the application is running in the browser. The client only needs to know what is different in the image as it changes, so although sending screen images sounds heavyweight, it is amenable to optimisation and compression.
It is the same concept as Windows remote desktop and terminal services, or remote access using vnc, but translated to a browser application that requires no additional client or setup.
There are downsides to this approach. First, it puts a heavy burden on the server, which is executing the application code as well as supplying the images, especially when there are many simultaneous users. Second, there are tricky issues when the user expects the application to interact with the local machine, such as playing sounds, copying to the clipboard or printing. Everything is an image, and not character-by-character text, for example. Third, it is not well suited to graphics that change rapidly, as in a game with fast-paced action.
On the other hand, it solves an immense problem: getting your application running on platforms which do not support the runtime you are using. Native applications, Flash and Silverlight on Apple’s iPad and iPhone, for example. I recall seeing a proof of concept for Flash at an Adobe MAX conference (not the most recent one) as part of the company’s research on how to break into Apple’s walled garden.
It is not as good as a true local application in most cases, but it is better than nothing.
Now, if Microsoft were to do something like this for Silverlight, enabling users to run Silverlight apps on their Apple and Linux devices, I suspect attitudes to the viability of Silverlight in the browser would change considerably.