Can't recall if this is mentioned but the ERSO (Erkeil Real Skyrim Overhaul) is very popular right now. it is very modular based so you can pick and choose what you want. Supposed to make the game much harder in many ways:

http://skyrim.nexusmods.com/mods/26217

I haven't tried it myself as I am very happy with the difficulty myself … then again I impose numerous limitations on my character that make even "Adept" a struggle to play on sometimes so I don't need any mods for difficulty.