I more or less paid for my computers selling a shareware¹ calendar program for Macs. It just let you make a one page calendar² of a month and type things in the days with some simple styling.
I initially wrote it as a birthday present for my brother, who needed something to do that job, then decided maybe some other people would like it. It was popular with teachers. (Writing that I feel like a jurassic patio11!)
I think it was $10, then I raised it to $15. These were pre-internet days, pre AOL even. It was fun getting checks from foreign lands, though it annoyed my bank. They eventually started charging $20 to clear a foreign check, so I had to adopt an unofficial “foreign folk get registered for free” policy and just thank them and tear up their checks when they arrived. I think the last check came in over 10 years after I stopped maintaining the program.
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¹ Slightly nag ware. Registered users could make the solicitation screen stop showing up on launch. There was a secret little click dance you could do on the solicitation screen to prove you were registered.
² Pro tip: A “calender” passes the spelling checker, but is a machine in which cloth or paper is pressed between rolls until smooth. You will have to update your software and reprint your paper manuals when you discover this fact.
Bonus Pro Tip: If you try to write a direct SCSI access disk utility, you may accidentally corrupt your only hard drive, not have backups of your source code, and then get to write a block scanner using only floppy based tools to scan the wreckage of your hard drive looking for blocks that might be C source code, collect those together, and reassemble your software sources as best you can. That's ok, version 2.0 is always better anyway, and you will have learned to keep backups.