Using C#, Visual Studio 2010.
There is a namespace called System.Web.Mvc documented on MSDN. The documentation for all the types in that namespace says that they are
In VS Express 2012 I couldn't find System.Web.Mvc in the "assemblies" tab, but after a bit of searching I found out that I need to look into "assemblies\extensions" tab rather than the default "assemblies\framework" tab.
The desired assembly has appeared in the list now.
I can only speculate what caused it to appear, but I suspect it is the fact that I went File → New → Project → ASP.NET Web Application, which I had never done before. It is possible that this caused some sort of late initialisation to happen and the list to be populated with additional assemblies for Web development.