First, creator Obelia medusa's Tiny Bag-End, Frodo's Hobbit-house faithfully recreated from the movie. Includes lots of nice close-ups.
Sal's Site has a sizeable collection of excellent dollhouse works by artist Sally Wallace, but be sure not to miss her Hogwarts. She has also done Mr. Ollivander's Wand Shop and Honeyduke's Candy Shop.
Bonus: not a dollhouse, but one of the most wonderfully detailed Lego structures I've seen: "Entrance to the Caves" by Norro. Full of little nooks and crannies, with some great landscaping -- those rocks! and those trees! These thumbnails don't do it justice -- check out the full gallery, with lots of different views.
Via The Brothers Brick.