Appreciate the responses, everyone.
My nomination is spreadsheets - I can go with “Excel” or “Visicalc-Lotus-Excel” or whatever, but in terms of productivity, usefulness, variety of output, and more, it’s hard to beat Excel.
However, TCP/IP and the *nix kernel are extremely strong contenders, as well as the C/COBOL/FORTRAN compilers mentioned.
Does HTML count? Wait! I’m the OP - of course it counts! 
Baron Greenback, apparently I wasn’t |337 enough (or I forgot) - what did adding the semicolon do?