WorldWideWeb, as it was.

[Dartz]

'Oldest' program I ever used in work was a mid-80's fork of an program that had been 'open source' before that was a term, had a working group instead and ran faster on an old Pentium four than a Core2Duo so it was highly single-threaded.

It took punch-cards as an input too. In the mid-2000's. Which translated to a meticulously organised and spaced notepad file instead,

The Alternate Transients Program was a mighty thing.,