@rajiv_ayyangar and I have talked a lot about what applications, devices, and codebases have most influenced our thinking and careers. Photoshop is high on that list for me.
For many years it was the most reliable application I used regularly, bar none. In a world where we just expected the Windows blue screen and the little Mac unhappy icon to happen regularly, Photoshop never crashed. Even though both its feature set complexity and working data set size were much larger than most other apps of the time.
Photoshop was also truly cross platform. It worked the same way on Windows and Mac. This is hard!
It had a plugin system. Indie developers built great extensions for Photoshop.
And it had a credits screen that you saw every time the app loaded, with the names of all the programmers who worked on it!