spreadsheet
Lotus Improv on the NeXT
"Cult classic" isn't a term typically applied to software, but if it were, this would be its poster child.
spreadsheet
Apple II
AppleWorks on the Apple II
1984 not only brought us the Mac, it brought us an 8-bit app that made it easy to resist purchasing a Mac.
spreadsheet
Lotus 1-2-3 on the PC w/DOS
VisiCalc started it, but 1-2-3 finished it. "It" being the discussion of what a spreadsheet can be, and also VisiCalc itself.
spreadsheet
VisiCalc on the Apple II
In 1978, Bricklin and Frankston took out a loan to develop "Calcu-ledger," a tool that could project the fiscal health of taking out that very loan