Time Out New York has an awesome list of the Always FREE museum in New York City:

• The Center for Book Arts
• The Forbes Galleries
• The Grolier Club
• Harbor Defense Museum
• The Hispanic Society of America
• Jeffrey’s on Essex @ the Essex Street Market: Cuchifritos Art Gallery
• The Korea Society Gallery
• Lefferts Historic House
• Leo Baeck Institute New York
• Little Red Lighthouse
• The Museum at FIT
• The Museum at Hebrew Union College—Jewish Institute of Religion
• The Museum of American Illustration at the Society of Illustrators
• The New York Public Library: Humanities and Social Sciences Library
• The New York Public Library: Library for the Performing Arts
• The New York Public Library: Science, Industry and Business Library
• Nicholas Roerich Museum
• Onassis Cultural Center
• Queens County Farm Museum
• Rose Museum at Carnegie Hall
• Scandinavia House—The Nordic Center in America
• Schomburg Center for Research in Black Culture
• Statue of Liberty & Ellis Island Immigration Museum
• Swiss Institute