How do I get the value from an anonymous expression?

You will need to call Compile() on the expression to get the Func and then execute that.

public string SomeFunction<TModel, TValue>(this HtmlHelper<TModel> html, Expression<Func<TModel, TValue>> expression)
{
    TValue valueOfBar = expression.Compile()(html.Model); // Assumes Model is accessible from html.

    // Do stuff
}

Side note: If there isn't any need for the dynamic expressions or expression analysis you might as well pass the Func directly in instead.

For those that are using expression without MVT Model, one would obtain name and value of property in a following way.

public static string Meth<T>(Expression<Func<T>> expression)
{
    var name = ((MemberExpression)expression.Body).Member.Name;
    var value = expression.Compile()();
    return string.Format("{0} - {1}", name, value);
}

use:

Meth(() => YourObject.Property);

Simply compile the expression and get the value.

Func<TModel, TValue> method = expression.Compile();

TValue value = method(html.ViewData.Model);
// might be a slightly different property, but you can get the ViewModel 
// from the HtmlHelper object.