A handful more games that didn't make the list for one reason or another.
A classic tile-based puzzle game among old-school Windows gamers, being distributed with several of the Windows Entertainment Packs back in the early 90s and having 149 stages to solve (as well as several fan-made level packs to add even more). However, it's technically ineligible by my rules, since the original release of the game was actually for the Atari Lynx three years earlier. Sorry!
(I could have put the two-decades-delayed Chip's Challenge 2 or its "spiritual successor" Chuck's Challenge 3D in their place I suppose, but... nah.)
A sandbox toy that showed off how powerful and flexible the Source engine really is. Not only could you spawn and play around with models, enemies, and basically every prop from all the major Source engine games, but people have crafted machinima animations, entirely new games and roleplaying servers and basically anything else you can think of utilizing complex rigs and Lua scripting. Really fun to mess with, but not a game per se, which is why I left it off.
Gamebryo RPGs (Morrowind, Oblivion, Fallout 3, Fallout New Vegas)
Though every major game Bethesda has released since the early 2000s has been on consoles as well, the PC remains my preferred way to play them. The reason why is pretty simple - modding support. Not just to fix the numerous scripting errors and bugs left in by the programmers (though some of those can be fun), but to add new quest lines, weapons, perks, vehicles, recruitable allies, and whatever else you may want. With thousands available to experiment with, all of these games become a fresh experience each time you revisit and replay value is basically infinite.
A game that recognizes that janky physics can be quite a bit of fun in their own right, Goat Simulator just puts you in the role of a goat with a sticky tongue and sets you loose on a city to cause mayhem. You get a ton of achievements for doing various things - finding objects, blowing up gas stations, knocking stuff over or just driving people crazy, but you're by no means bound to any goals. The devs even recognize that general engine weirdness can be fun, as it was their policy to only fix game-crashing bugs and leave the rest intact. They even added more modules parodying MMORPGs and the Payday games (having you prank people as a team of animals with various ridiculous talents), so if you just want a bit of good, mindless fun, Goat Simulator is the place to find it.
The Hugo trilogy
Another one that made the rounds in the era of shareware, the Hugo trilogy was notable for being a Sierra-inspired adventure series designed and coded entirely by one guy. That's all fine and dandy, but it falls prey to some strange design decisions, particularly in the second game where you have to walk between obstacles with unclear hit boxes and even the slightest touch will kill you or leave the game unwinnable. They're not particular standouts of the genre or anything, but they are a bit of fun for what they are.
Mortal Kombat Trilogy
A multiplatform release from the tail end of Mortal Kombat's 2D era, its selling point was including everybody from the first three games (even the bosses) as playable characters. It did just that, but no real playtesting was implemented either, so it was a ridiculously unbalanced mishmash of nonsense; that's also what made it fun, though. The console ports all had some flaws - the N64 version lacked load times but had poor sound quality and several fighters missing owing to cart space restrictions, while the PSX and Saturn versions had everyone and higher-quality sound but rather atrocious load times (particularly in Shang Tsung matches, where the game would pause for several seconds and load mid-match every time you morphed). The PC version, however, was arguably the definitive release, having all the characters, high sound quality and, since it installed everything to disk, no load times either. So if you want some dumb, campy fun, track this one down and give it a play!
If you've been on Youtube watching gaming videos for any period of time you've probably seen at least one of MUGEN, a fighting game engine that lets you import your own graphics, build your own characters and tweak nearly all of its mechanics to your liking. Because of this, people have made literally thousands of characters to play around with - some attempting to be legit recreations of existing fighters, others original, and still others just meme-laden broken nonsense. People have even included stages from other games like Super Mario Bros as "fighters", so it's surprisingly flexible. But you really do have to put in quite a lot of work and hand-tweaking to set it up and it's not really much of a game in itself, so I opted against including it.
Portal, Portal 2
I do love me some Portal. A twisted action-puzzle game set in the half-life universe where you utilize the titular portal gun to sneak past obstacles, hop over long gaps, bridge two areas that otherwise wouldn't reach and do all sorts of other clever things to make your way to the end. All presided over by the twisted AI GlaDOS, who is both threatening and consistently hilarious. Unfortunately both games were also simultaneous releases on consoles and therefore disallowed by the rules I set.
Simcopter/Streets of Simcity (Maxis, 1996/1997)I mention these two not because they're good; quite the opposite in fact. They're very ugly early 3D games that have mountains of unpatched bugs, esoteric coding that makes them almost impossible to run on anything but very specific hardware from the early 90s, and even then they crash frequently, sometimes bringing the entire OS down with them. But you know what? That can be pretty fun sometimes. As long as you're not set on taking them seriously, playing around with their insane physics models and uniquely janky mechanics, topped with Maxis's strange sense of humor, some unironically good soundtracks and a rather controversial Easter egg in Simcopter inserted by the programmer as a prank, makes these games surprisingly entertaining even with their enormous flaws. The guiltiest of pleasures, these.