There are tons of excellent pieces of Apple software that come with iOS[EDIT macOS, that is] (or are free to download).
Preview's simply amazing and I miss it very much on every other platform, including iOS, but probably doesn't count as a hidden gem.
One that took me way too long to discover was the Digital Color Meter. It's a color picker that comes up instantly (much of what's so great about Apple's software is that they give at least half a shit about performance, unlike seemingly everyone else these days—it shouldn't be impressive that a color picker window comes up instantly from a cold start, but in the current software environment, it kinda is) and works for everything on your screen.
Preview's simply amazing and I miss it very much on every other platform, including iOS, but probably doesn't count as a hidden gem.
One that took me way too long to discover was the Digital Color Meter. It's a color picker that comes up instantly (much of what's so great about Apple's software is that they give at least half a shit about performance, unlike seemingly everyone else these days—it shouldn't be impressive that a color picker window comes up instantly from a cold start, but in the current software environment, it kinda is) and works for everything on your screen.