The infamous San Pedro prison (El Penal de San Pedro) in La Paz is famous for having no security guards inside, the prisoners run it themselves. Families of the prisoners often live there too, and it's apparently possible to do 'tours' of the prison, also run by the prisoners. However it's hard to find information on this as no official tour company can be seen to endorse these, and I'm quite sure nobody would insure them either ;)
The tours of the prison used to be possible in the past, but with the publication of the book 'Marching Powder' they became very popular and the government finally cracked down on the whole situation in the prison.
It is no longer possible to do the tours or get into the prison (unless you are convicted to serve time there)