Wash cloths and sponges
It's very unsanitary to use someone else's wash sponge, cloth, loofah or any other item they use to clean themselves.
It's very gross to think of all the 'personal' bacteria and body fluids you could be transferring to yourself plus, doing this creates a perfect environment for an infection to occur.
So save these for your use only and if you have guests over often, have a pack of new ones in case someone forgets to carry theirs.
Underwear
An interesting survey exposed that one in four couples share underwear. Some people have even admitted to sharing their underwear with their siblings and others have no problem buying second hand underwear.
The number of risks here are too many to count. You don't know what the other person is dealing with 'down there' so to be safe, don't do it.
Just remember that anything that comes into contact with your intimate areas shouldn't be shared with anyone else and this includes bathing suits.