Starlancer (2000)

Rally Trophy (2001)

Rise of Nations (2003)

Rise of Nations: Rise of Legends (2006)

Take no Prisoners

cool top down shooter

It's unfortunate that it never had a DOS release as that sounds like an interesting game.

If this shouldn't be limited to adventures, and taking old as meaning before 2000, let's see what wasn't mentioned yet...
Stunts and SimTower would fit. And maybe Holiday Island too, at least for a short stint.
And as a bonus, the original Transport Tycoon Deluxe.
Would have a few more if I can go a little past 2000, but let's stop here now.

Alternate Reality
Electronic Popple
Terminator 2029
The Colony (Amiga)
Wings of Fury (Amiga)
Desert Fox (C=64)
Defender of the Crown (C=64)

Some games I play that are not currently sold anywhere.

No Man's Land ( Age of empires style RTS )
https://www.mobygames.com/game/windows/no-mans-land

Starship Troopers: Terran Ascendancy
https://www.mobygames.com/game/windows/starship-troopers_

Conflict Zone
https://www.mobygames.com/game/conflict-zone

Nuclear Strike ( PC version is the best )
https://www.mobygames.com/game/nuclear-strike

Terror Strike
https://www.mobygames.com/game/windows/terror-strike

War Front: Turning Point ( Alternate WW2 RTS with Generals Style gameplay )
https://www.mobygames.com/game/windows/war-front-turning-point

The Thing ( The real sequel )
https://www.mobygames.com/game/thing

Vietcong
https://www.mobygames.com/game/windows/vietcong

If you want C64 version specifically (and there's nothing wrong with that!), then it's unavailable.

But otherwise it's available on Steam in a collection called "Cinemaware Anthology: 1986-1991".

I'll not write a endlessly long list of Amiga or C64 or old Console games, since these are platforms that just don't exist anymore.
So on the PC we stay

Like everyone else I fist must nominate:

No One Lives Forever
No One Lives Forever - A Spy in H.A.R.M.'s way
Self explanatory. Specifically excluding Contract Jack from the same series, that was garbage.

Heav Metal F.A.K.K. 2
Play Julie Strain in a 3rd person shooter that has it all.
It came out shortly after the movie

Heart of Darkness
An excellent 2D platformer by the people who also created Another World. It has a full orchestral soundtrack as well.
Development time took way too long, so when it came out, the graphics were outdated. But the game was brilliant.
Also came with blue/red rd glasses, so the intro sequence could be watched in 3D as a bonus.

Space Bunnies Must Die!
3rd person perspective action game, nice humor.

So Blonde
Point & click adventure. A blonde it-girl who cares more about the color of her finger nails than about getting something to eat, stranded on an island, having to deal with pirates.

Croc - Legend of the Gobbos
3d platformer
Great game. It also used the Soundblaster AWE to play background sound, making it more vivid without CPU cost.
When Windows locked direct sound card access however, those sound would not play anymore. Apparently a later DirectSound version of the games has the sounds restored.

Gexx 3D Enter the Gecko
3d platformer. I was never really good at it, but it was fun none the less.

Desert Strike
Isometric shooter: Fly a Apache, shoot bad guys, resque brave soldiers, 'manage' fuel and ammunition.

Jungle Strike
More of the same with different graphics, the CD version also features FMV cutscenes.

(for me unavailable)
American McGees Alice
the predecessort to the still available game is not being sold in Italy for whatever reason. Also packages containing that game are banned

Boiling Point
Before Far Cry 2, there was Boiling point. You would play a ex soldier on the search for his daughter on a tropical island.
There are several factions who fight each other, so you also had either to pick sides or play them against each other, so it hat certain role play elements.
The game was way beyond what every other compareable game from that time offered. But sadly it was also a bug feast. Constant crashes took the fun out of it so people forgot about it pretty quickly. Would have been nice if it stuck around and got fixed.

Manhunt
over (not ultra) violent reality TV show where people must hunt one lonely guy ... guess who you play.
Very very good Stealth game. If it hasn't been banned in several countries, it would have give Splinter Cell and Thief a run for their money.

Bazooka Sue
Good, but not great. Comic point & click adventure, road trip style. You play a pig with two big ... personalities, who doesn't take shit from anyone, stranded in a small town.

The Riddle of Master Lu - RIppley's Believe It Or Not
Excellent old style point & click adventure, LucasArts style, but a bit more on the ground. The main character existed in real life and was a guy who was searching worldwide for curiosities.
All characters are filmed real life actors. OF course that brings a few limitations when it comes to transitions between animations, but it works really well.
This is a permanent inhabitant of my DosBox games folder.

The X-Files
FMV point & click game on 7CDs (or one DVD). Mulder and Scully are vanisheshed, the FBI sends you to find them.
A lot of people were disappointed that we could not play Mulder or Scully, but the game is great anyway.
It still runs with a quicktime emulator, sadly with minor display bugs. The bars on top and bottom are not black as they should but grey and not always hide the icons that should not be visible during cutscenes.

James Pond 2 Codename RoboCod
Excellent 2D platformer. Thanks to it being released only in 1993, it also supports Sound Blaster music, therefore sounding almost as good as the Amiga version, also scrolling is a LOT better than in early VGA games.

Touche: The Adventures Of The Fifth Musketeer
Another great point & click adventure. Runs in ScummVM, has a few minor issues with cut off sound.

And last but not least
Turrican 2 The Final FIght (PC)
This is how a conversion has to be done. It plays exactly the same as the Amiga version, also features it's sound track.
The graphics have been remade to support VGA colors. Some people might complain about not having the same graphics as the original, but that's what we have emulatoras for. The redesign by Sonflowers is excellent (except for Turrican's head maybe). Music/sound and scrolling and excellent, finally someone who knows how to make VGA work properly in 2D games.
It's however made for a analog monitor, so to make it run in DOSBox, it's necessary to delete the graphic informations from the configuration file, so it has to run the detection - every time. So to run the game, I made a batch file that replaces the current configuration with another one before every start.
Best played with a Speedlink Competition Pro joystick.
Back in the days the main problem was to free enough RAM to run the game, with DOSBox that is not an issue.

Biosys is a favorite point-and-click game I enjoy.

https://en.wikipedia.org/wiki/Biosys

Steam have a game called that, but it is a side scrolling platform game and not the following Top Down game I enjoyed many years ago.

https://en.wikipedia.org/wiki/Take_No_Prisoners_(video_game)

I know it's not available. That's what the topic is about. That's why I mentioned it. :)

The original version of CHASM: The Rift is not currently available to purchase on its own, though you can get it as a bonus if your purchase the much more expensive revamped version.

Yeah, I thought that was the point.
But it's also true that if you search Steam using the game name, nothing will show up, but it's actually there in a collection.

And now I actually notice that it's also available on GOG!

https://www.gog.com/game/defender_of_the_crown

I was under the impression that it wasn't here for some reason, but apparently it is.


But the 2002 and 2003 remakes seem to be absolutely nowhere, unless I have missed something obvious.

So many Windows 3.x and 9x games.
Such as, but not limited to:

Gravity Well (1995-1997) (Monounit RTS)
Stars! (1995-1996) (Classic Windows 3.x 4X.)
Moraff's ______ (Plethora of years) (There's a bunch of games, I can't describe them all.)
Neural Ned in NED's WORLD (1993) (Yes, someone was making neural networks in 1993)
Razzle Dazzle (1990-1997) (An old DOS screensaver.)
Microman (1995) (Brian L. Goble's early foray into game design.)
The Inside World (1995) (An interesting dungencrawler where you play as average people vs giant bugs.)
Daisy's Garden (1994-1995) A softer, gentler Lode Runner clone made in Germany.
Comet Busters! (1994) A very cool bitmapped asteroids clone with some interesting twists.
Dominate! (1993) A "control the board" game that would be otherwise unremarkable if not for custom styling and sounds.
Photon Chess (199?) It's that chess variant with the laser piece, mirrors, and such. Neat!
Chargeball. (199?) A Swedish game about attracting or repelling electrons and photons into a goal using only mouse control.
Wormworld (199?) A weird puzzle game that's like a monounit lemmings; about guiding a bright green worm about.

Forgive me if it has been mentioned before, but Isaac Asimov's Robot City.

I think not long ago Scummvm announced support for another game from the same developer, so I hope, in the not so distant future, they offer support for this once as well.

It's a childhood favorite of mine and I still have the CDs. :)