I thought the tool that increased the draw distance, removed the fog and added distant land rendering incredible. You can actually see the landscape stretch out towards the horizon with it, like in Oblivion. Of course, your frame rate will suffer some from this.

I don't remember what the tool was called.

UPDATE: Okay, the tool is Morrowind Graphics Extender and it's really fun to play around with. Some videos and screenshot of it in action.