Oldest ones are all DOS:
Newkey - best shareware keyboard macro program for DOS, about 1990.
Early Bird - reminder program for DOS, also about 1990. Still use it a bit (Y2K-compliant from the start).
QEMM/DESQview - commercial - plus NDOS, best system I ever had.
Blue Wave OLR
GoldED message editor for Fidonet
Visual Display Editor (VDE) - Wordstar-style editor/word processor, still updated, still use it
PC-Write word processor, still use it
XTree Pro, long since abandoned in favour of a succession of Norton Commander clones.
Interesting point. Newkey, which I registered in 1990, was the first program I registered. I used it because I wanted to automate things, help certain programs talk to each other, put my own acceptable user interface on the results, and generally put sticking plaster (Band Aids?) over a poor OS that didn't want to be as automated as
I wanted, and application programs that didn't work like I wanted either. Times changed, computers got bigger, programs got more sophisticated. Multitasking arrived courtesty of DESQview, and Newkey and DV wouldn't play nicely together. After a bit of searching I found new ways of working, abandoning Newkey with respect and regret. Later still, Windows arrived and spoiled everything. The first Windows program I registered, in 2000, was Macro Express. I used it because I wanted to automate things, help certain programs talk to each other, put my own acceptable user interface on the results, and generally put sticking plaster (Band Aids?) over a poor OS that didn't want to be as automated as
I wanted, and application programs that didn't work like I wanted either. Ten years between registering my first DOS program and my first Windows program, and Microsoft made sooo much progress...