The Great Learning, by Confucius. (This site has other great etext classics as well, including Ulysses.)
Embedded ML allows functional programmers to compile functional programs that can be run on small and embedded machines.
Ledger is a powerful, double-entry accounting system that is accessed from the UNIX command-line. Very useful once you got it.
Curt Herzstark and his Pocket Calculator CURTA, history and origin at The Calculator Reference.
Semigroup/Group proof in Coq.
Acid Trips, a tutorial talk for acid(1), the programmable Plan9 debugger. By Russ Cox.