I like doom 3, but it is starting to show its age. I found this mod, which is a collection of MANY mods and puts them all into one very easy to use, install package. It adds all new textures, sound overhaul, all new animations, you can now aim down sights, Ambient occlusion and gives you all the tweaking options you could want. It also makes the baddies tougher.
Here is a video of the mod in action (Not my video)
http://www.youtube.com/watch?v=ajW9LTVTRNg