Yup, hessies is right. I have to say that is one thing that I thought was rather handy when I tried 3DS Max (before I tried to model something with it and got nowhere). What I mean is a render taken from the viewpoint of whatever viewport you are in, and rendered in an external window, just like a normal render is.
As it is now, the quickest way that I know of rendering what you see in your viewport is to set the camera to the active view using ctrl+alt+0 and then physically grabbing the camera and trucking it back in order to achieve the view that you were in before.
In essence, what I mean is, why can’t the viewports behave like virtual cameras. To render from the ‘camera view’ of the viewport, you have either a function key, such as Ctrl+F12 or something similar. Whichever viewport is active, will be the one that produces the render.
Since viewports are variable sizes, it would most likely need to have a setting where you could define your max width and height for whatever the output is.
Also a thought: being able to ‘pin’ your renders so that they remain open when you press render again would enable you to have multiple render windows open for comparison.