I was always put off by the fact that, whatever the time or place, NPCs always wear the same clothes. Follower NPCs especially mostly stay in their battle gear, even when they go to bed! About the only NPC I can see who changes her clothes is Serana, who changes back to her vampire gear once you tell her to stop following you. Is there a mod that takes control of what an NPC wears depending on what time of day it is, whether they're indoors or outdoors etc.?

Wet & Cold has the option for NPC's to wear hood when it's raining and fur hoods/cloaks if it's cold/snowing.

Some mods do this for everyone, which can cause problems - NPCs might choose to wear nothing rather than their armor if they have no other clothes; fine for sleeping, a little disturbing on the streets of Solitude.

For followers, some of the major follower extension mods sort this out:

You can ask NPCs to change clothes with http://www.nexusmods.com/skyrim/mods/12933 - I don't use it anymore myself so I can't recall if you can make it automatic.

http://www.nexusmods.com/skyrim/mods/15524/: automatic helmet removal, automatic outfit change at home & in cities.

Vilja changes into a nightgown when she sleeps and armour when she enters combat. Other times, she wears a suiable dress - with cloak and/or hood if you give them to her.

Vilja's wardrobe system is by far richer and more complex than that...?

Yes, that's what I had in mind, characters who have a bed costume, a home costume and a duty/battle costume (especially when you take them on a mission).

I'd love this. I'd also like to have NPCs just change clothes once in a while so they don't wear the same clothes forever and ever.