Grid Cartographer

I love mapping my video games. In my childhood, I would draw maps of the mazes you'd always get in Sierra point-n-clicks. And when replaying a game, I'd draw its map all over again, just for the fun of it -- even when I still had my old map available. There's something very satisfying about slowly documenting all the twists and turns of an area until you have the full picture. It feels triumphant, even.
Drawing on paper is fun! But you're bound to make mistakes, and trying to correct too many of them will just make the thing a huge mess. That's where Grid Cartographer comes in. It's powerful but simple to use, and does exactly what I want a digital mapper to do. But it's pricey!!!
The high asking price is justified, considering the advanced features this piece of software has -- even though I'll probably never ever use them. The big one is the GameLink feature that lets you automatically track your position on your map as you play certain old-school dungeon crawlers. Grid Cartographer is still actively supported by its dev, too, despite being almost eight years old now, which is a pleasure to see. There are freeware alternatives, like mipui.net or Gridmonger, but they're either lacking in crucial features or are much more difficult to use.
Having said all that, I'm not completely 100% happy with GC. The set of markers feels limited, especially if you're not playing a fantasy title. You can import more markers, but they're not readily available to grab from anywhere -- I only found a few individual ones on the Grid Cartographer wiki. You end up having to draw your own. Also, I wish there was a way to draw a corridor adjacent to an existing corridor without having to re-draw the wall between them every single time. (But you can use the corridor brush to draw the walls first and then just fill them in, which is marginally easier)
And when a game uses anything other than a neat rectangular or hexagonal grid -- it's back to good ol' paper for me ːkleinesː

I think this is a great tool for mapping old CRPGs and even has some particularly nice game link features in that regard, but for the purposes of creating your own TTRPG map I'd say it is very decent and can produce nice-looking maps with very little effort, but it is also missing a couple of features.
Most notably, despite scouring the interface and searching online I can't find any way to make circular rooms beyond a certain size, which seems like a big thing to be missing when even free browser-based mapping tools often have that. I think that and lack of variable size options for things like brushes and markers are the main things I'd appreciate in future updates (if there's any more coming).
More markers would also be nice but more options for non-blocky rooms is really the main thing holding this program back in my opinion.
If there is in fact a way to make true circular rooms besides just putting four half-blocks together (which limits them to a specific size) then please let me know.
Edit to add: I have no idea why but it seems to be running horribly on Linux. It's so laggy it's not really functional anymore.

An incredibly useful tool if you like to play old crpg's (you know who you are). Also very hand for designing maps if you make games yourself.

tl;dr: A very capable piece of software serving a niche that goes shockingly unfulfilled when you start looking around for options. Probably wildly overpriced for most people unless it's on sale, but considering it's closest market contemporary is a ♥♥♥♥♥♥♥ subscription service, this actually ain't half bad.
My particular use case for Grid Cartographer is mapping in dungeon crawlers that do not have built in mapping, so that's the kind of direction I'm coming from it at. Basically just graph paper with an undo button. I'm not using it to create or run TTRPG capmaigns or anything like that, although it has the capabilities to do that too.
It has three core "types" of things you can put on a map: Terrain, Edges, and Markers. Basically floors, walls, and "stuff". It has a fairly robust selection of tiles and icons that will cover most of the common things you might find in a dungeon or whatever. I don't doubt it has blind spots, but it's a really good baseline. Of course you can import your own if there's something it lacks or you simply want things to look a specific way, and all of these can be colored as desired.
It has about a dozen default "themes" for the maps, mostly variations of well recognized two-tone color schemes for things like graph paper or vector monitors/PC terminals, although there is also one that mimics parchment. All of these can be tweaked, and you can make and save your own themes if desired.
It supports multiple floors and "regions", which are sets of floors, and even has the ability to automatically create linked stair tiles on upper/lower floors when you place them. It also has a variety of image editor style tools to do things like rotate/copy/move tiles you've marked with a variety of selection tools, even including a smart wand. You can add notes to tiles, as well as a text tool to label things. Also supports Hex tiles in both orientations (side or point facing up), and includes some elevation tools to set floor/ceiling heights if that's something you need. All in all it's very capable while also being easy enough to use as a simple graph paper replacement.
It has some gimmicky stuff like the ability to explore your maps in first person and track your footsteps, or even link up to emulators or other kinds of running games and track inputs to act as a faux automapper for games that don't have them. Interesting stuff, but real fringe and I haven't really explored that angle of it.
So I guess the conclusion is that if you want Photoshop But For Maps, then this is kind of it. A fully-featured editing software that specializes in grid or hex-based maps for dungeon crawlers or TTRPGs. You think this kind of thing would be more common, but the offerings are somewhat pitiful so this clears basically anything I've seen, and I kind of went down a rabbit hole looking for a few days.

Fantastic mapping tool.

Excellent. Perfectly done. Every detail is thought.