The Shutter Island star purchased the New York townhouse with Ledger when the former couple first found out she was pregnant with their daughter Matilda, now five.
Williams now keeps an open-door policy at the property, admitting she surrounds herself with house guests to distract her from feeling sad.
She tells Britain's Telegraph magazine, "(The house is) too much, too sad, too enormous... Every time I come home, there's a new note on the refrigerator (from a guest) saying, 'Thanks for letting me stay here.' It staves off the loneliness, or the desire for a relationship. When I get home I can get into bed, read a book, talk about my day."
Williams and Ledger fell in love while shooting Brokeback Mountain in 2005 but they split in 2007. The Dark Knight star died of an accidental overdose in January, 2008.