I believe these two are popular:
http://skyrim.nexusmods.com/mods/26228/?With this one you need to eat, sleep, and drink. I havent played with it but I've heard it is done in a good way where it doesn't just become a chore like some realism mods.
This is another extremely popular one:
http://skyrim.nexusmods.com/mods/11163//?With this mod you have to pay attention to the cold. Being cold can reduce stats or lead to death. Certain areas are colder than others, the type of weather matters, how much and what type of gear matters when it comes to your exposure level. You are able to construct campfires/tents to help reduce the cold