The Joseph Smith Homestead is located at the Foot of Main Street on the West site of the street. It is owned by the Community of Christ Church and you are able to tour the house with a ticket, sold in the Red Brick store.
The log portion of the homestead was standing when the Saints first arrived in Nauvoo in 1839. It was probably used at the first Indian agency in Illinois before the Saints arrived, they believe it was built in about 1803. Joseph Smith added the north room in 1840 and Joseph Smith III added the large portion on the west side of the original structure. The summer kitchen in the back of the Homestead is a reconstructed log cabin on the original site. The log cabin was lived in by Joseph Smith Sr. and Lucy Mack Smith. Joseph Smith hid from enemies in a secret place under the house cellar stairs.
It was from the homestead where the prophet Joseph Smith rose from his sick bed and went and administered to the sick, when the Saints had come down with malaria from the misquitos in Nauvoo. The prophet started administering to the sick in his own house and dooryard. Then the prophet walked along the Mississippi Riverbank to the Lower Stone House "healing the sick on every side". He then went to Montrose and did the same there. It was July 22, 1839 and became known as "a day of God's Power". (More on this store here).
The Egyptian Mummies were stored for a time in the upstairs of the Smith homestead. Also, sometimes sacred temple ordinances were performed there.
More Things to see in Nauvoo
References: "Sacred Places Ohio and Illinois, a comprehensive guide to early LDS Historical Sites." by LaMar C. Berrett General Editor and Keith W. Perkins and Donald Q. Cannon. p 128