The return of Bonehilda
Edit: This article details my experience with Bonehilda. Apparently there is a lot of variation in behavior among Bonehildas - check the comments for another Simmer's experiences, which were very different from mine.
Having trouble keeping up around the house? If you have the Supernatural expansion pack, Bonehilda can give you a hand! Sort of. Bonehilda began as a service NPC in the Sims 1 Makin' Magic expansion pack. She returns to The Sims 3 with a very similar set of advantages and drawbacks.
You can find Bonehilda in Buy Mode under Décor -> Sculpture. Buy her living quarters (an upright casket) and set it on her property. Then click on her casket and have one of your Sims choose the "Awaken Bonehilda" action.
Unlike the other service NPCs, there is basically no interaction between your Sims and Bonehilda. Apparently they can chat with her, although I have never had that option. You cannot build a relationship with Bonehilda (thus she can't be married or impregnated.) I can only ever Dismiss Bonehilda, which sends her back to her casket until you awaken her again.
Bonehilda has an interesting role. She seems to do best as a nanny. When I had a baby in the house, she served as the primary caregiver. She held him whenever he got lonely, changed his diaper when it needed changing, and made sure he was always fed. When he became a toddler she would feed him and ferry him to and from his crib. However, she never talked to him or played with him, much less tried to teach him one of the Big Three (walk, talk, and use the potty).
She will occasionally clean up some dirty dishes, but I have never seen her picking up the laundry. And unlike the Executive Butler, I have yet to see her make a meal.
Bonehilda also spends a lot of time attempting to interact with animals. I had a cat and a horse at the time, and she was constantly following them around trying to pet them. They hated her, though, so it always ended up in a negative interaction. Although I did see her fill the cat's food dish once or twice, which is more than she ever did for my people Sims.
Non-household Sims may be frightened of Bonehilda. I have a friend who reports that she tried to hire a maid, in order to compensate for Bonehilda's deficiencies in that regard. But the maid quit, citing her terror of Bonehilda as a reason. (I have never seen a maid quit before!)