Yes, you can write C# and get it to run in any browser which is something of a surprise. Miguel de Icaza has some good ideas about how this magic can be achieved.
This isn't exactly news, but it deserves to be better known.
A recent blog post by Mono's Miguel de Icaza discusses the idea of running C# programs in the browser - any browser. At a time when Microsoft doesn't seem as keen on all things .NET, Mono seems to be leading the way in extending its reach. This is a good idea for Mono because the more C# can be used the more it is likely that the Mono software will survive and flourish.
Until recently one of the areas that C# has been absent from is the client-side browser. It is used on the server side in ASP.NET and it appears to be used directly in web pages as part of the ASP.NET MVC approach. However, in each case the code is run on the server and the results shipped to the browser as HTML.