All Things Zen: Koan of Zen master Ch'ing Yuan Wei-hsin on emptiness, enlightenment, dependent origination, satori.
Awakening 101: A free online Dharma course that explores the Enlightenment experience in the Zen tradition, presented by the Wanderling, an anonymous, uncertified Zen teacher.
BuddhaNet File library: Dozens of .zip files of Zen teachings
Essentials of Buddhism: Lists out the key points of Buddhism in a nutshell.
Fukan zazengi: Translated by the Stanford Zen Translation Project.
Hakuin: "In the realm of the thousand buddhas He is hated by the thousand buddhas; Among the crowd of demons He is detested by the crowd of demons. He crushes the silent-illumination heretics of today, And massacres the heterodox blind monks of this generation. This filthy blind old shavepate Adds more foulness still to foulness."
How to Practice: The essential idea of Zen practice.
Master Hsu Yun's Teachings: Dharma discourses by a historical Chinese Ch'an master.
Soto Zen Ancestors in China: The recorded teachings of Tang-dynasty Chan masters Shitou Xiqian, Qingyuan Xingsi, Yaoshan Weiyen, and Yunyan Tansheng, translated into English.
Teachings of Bodhidharma:
The Ten Precepts: Dai Bosatsu version.
What do you think, my friend?: Selection of writings on Buddhism
What The Buddha Said:
Zen Master Seung Sahn's Temple Rules:
Zen Texts: A large miscellaneous collection of Zen Texts.