The Wumenguan, or Mumonkan in Japanese, known in English as the Gateless Gate, is a famous collection of 48 koans, with commentaries and verses by Mumon Ekai (Wumen Huikai), a 13th century monk who was abbot at Ryusho (Lungxiang) monastery.

Modern collections also include additions and commentaries by other authors, most notably Muryo Souju (Wuliang Zhongshou), and Tei Seishi (Cheng Qinci), who added a 49th koan.

Unfortunately, I have only come upon English translations of the Japanese or the Chinese versions, but not the original Chinese. I'd be much obliged if someone can point out a public domain copy to me.