This dll takes some of the pain away from registering routes in Orchard (ie by implementing IRouteProvider and manually adding in all the routes)
It's on Nuget as "Orchard.Routing"

So instead of registering routes using an implementation of IRouteProvider
and manually having to building RouteDescriptors etc,
you can just implement OrchardRouteProvider like so : 

    public class Routes : OrchardRouteProvider
    {
        public override string AreaName
        {
            get { return "Acme.Reports"; } // the name of your Orchard Module - ie the MVC Area
        }
    }

It then autoregisters your controller routes, according to how you've tagged them (see below)

And then tag your controllers : 

public class FooController : Controller 
{
	[Get("foo/{someParam}/bar")]
	public ActionResult Bar(string someParam) {
		// whatever
	}
}

There are attributes for Get, Post, Put, Patch and Delete
The attributes behave like the normal HttpGet, HttpPost (etc) attributes - ie if you tag an action as Get(), then you can't post to it

You can use them in conjunction with the normal Http* attributes, for example :

public class FooController : Controller 
{
	[Get("foo/new")]
	public ActionResult New() {
		// whatever
	}

	[HttpPost]
	public ActionResult Create(Bar bar) {
		// whatever
	}
}

If you don't use the HttpGet / HttpPost attributes for your actions, you can still register routes like so :

public class FooController : Controller 
{
	[Route("foo/{id}")]
	public ActionResult View(Guid id) {
		// whatever
	}

	[Route("foos")]
	public ActionResult List() {
		// whatever
	}	
}