There's probably a lot more foundational papers that are important, but these are my personal favorites.

As We May Think - Vannevar Bush, The Atlantic Monthly, July 1945. (or ACM DL link here)

The Science of Design - Herbert A. Simon in The Sciences of the Artificial, 1969, pp. 128-159.

User Technology: From Pointing to Pondering - Stuart K. Card and Thomas P. Moran. ACM conference on the history of personal workstations 1986.

The Computer for the 21st Century - Mark Weiser, Scientific American, September 1991.

Beyond Being There - Jim Hollan and Scott Stornetta. CHI 1992.

Why are human-computer interfaces difficult to design and implement - Brad A. Myers, Technical Report No. CMU-CS-93-183. Carnegie-Mellon Univ 1993.