How can I get C# to interop with Javascript?

you can also use a webbrowser control to host the javascript in a html document, to interact between the two you would make a COM visible class and set an instance of it to the ObjectForScripting property.

Any public members of the 'external' class are now accessible through window.external in javascript. Also, from the managed code side you can use the InvokeScript method of the document to call any javascript functions.

This way it is easy to pass complex objects between the two.

I used this idea quite a bit in this Google Earth Api application. http://fraserchapman.blogspot.com/2008/08/google-earth-plug-in-and-c.html

I'd highly recommend you use the Microsoft Dynamic Langauage Runtime (DLR). It's purpose in life is to facilitate scripting in a .Net environment. The DLR is not script specific so over time more script languages will be built on top of it but at the moment the ones I know about are IronRuby, IronPython and JScript.