Midnight mansions can be divided into three categories.  First, there are the mansions that come with the game, i.e. the included mansions.  These are 24 mansions from Falcon Manor easy to Spider Palace hard - and of course the tutorial mansion.  When playing the 24 included mansions you can aim for the highest score and the top ten lists at Actionsoft's website.
Second, we have the approved custom mansions.  These are mansions built by MM players and tested by other MM players for building flaws like Jack stucks, half Jack stucks, cheap deaths etc.  Only after serious testing are these mansions accepted and uploaded to this forum.  You can access the (around 115) approved custom mansions here.
Third, there are the free uploaded mansions.  These mansions can be accessed here and are mainly experimental products and unfinished stuff.  Any registered member of this forum can upload these mansions and they can be download here.  Note that these mansions have not been tested, at least not to the standards of the approved mansions.