Another gem from Daniel Hölbling.
In short – it will allow you to write code like:
public void Browse([DefaultValue("beer")] string category, [DefaultValue(1)] int page)  { 
...
}
grab it here
The cool thing is that because MonoRail is so extremely flexible, one can really easily add this type of functionality without touching the code-base, but rather implementing a straightforward interface. That’s what I call extensibility.
One more super kudos to Hammett for the overall architecture of MonoRail.