Not sure if games count as software but if they do - Factorio. I don't play much these days but I'm still utterly stunned how a relatively small, humble team of developers can build something so robust and performant. Granted, I've never really tried pushing the limits, but not once have I felt like the game is even breaking a sweat while processing thousands of machines, belts, and bots. It's a miracle to me.
Their devblogs are really nicely written and you can tell they are extremely passionate about getting things right. In my experience that's a rarity now.
Similarly, mindustry is written in java by a single developer freshly out of college and is incredibly performant for the amount of moving pieces rendered at a time. Granted I'm not in video games, maybe it's not too hard, but I thought it was really cool.
It's also an incredibly cool, interesting game design. If you like Factorio + Tower Defense you should check it out.
Doubly so that the game is written entirely in native C++ and is fully cross platform compatible (mac/windows/linux) with a tiny binary size. The responsiveness and performance of that game even with thousands of agents onscreen has always impressed me.
I'll also point out that the UI for Factorio is almost among the best.
I'd prefer a factorio over almost another UI.
I'm imagining a port of factorio's UI into most strategy games would be very nice, both from a VERY zoomable map, a clear research/progression tree. A few improvements could be made IMHO, but it's lighyears better than TF, CS, the Chris Sawyer set, etc.
There's a series of space strategy/4x/combat sims called X where the latest entry seems to simulate entire economies and individual ships in real time. Dozens of sectors with hundreds of ships and dozens of space stations all humming along at 30-60 FPS.
Like Factorio it's incredible and makes me wonder wtf I was doing chaining together bullshit service APIs in my day job.
Another elegant game is [1] City Skylines. It's amazing how they can create such a huge virtual world with millions of simulated individuals that roam around and have complex lives in your city.
Factorio really is a bit of a modern marvel, even HUGE factories don't seem to run poorly at all and I genuinely have no idea how. Other similar games made since (Satisfactory comes to mind) seem to struggle with things like keeping track of resources on belts at scale, which IMO is a perfectly understandable problem to have but still makes factorio that much more impressive.
You can definitely hit the limits of Factorio. My largest base runs at ~10fps which is how I know it's time to start a new game. Egregious use of copy/pasting and an automated robot production will get you there.
It's still leagues ahead of any other game I've played though.
I've definitely pushed the limits of performance with way too upgraded huge range artillery turrets hitting all the biter bases, but yes, very performant with large factories otherwise :)
Their devblogs are really nicely written and you can tell they are extremely passionate about getting things right. In my experience that's a rarity now.
And of course, it's a brilliantly addictive game.