Play abandonware games
Technically, abandonware games are games not supported anymore from their developers. If the original developer company is no more and if the game is so old that no one cares for it anymore, it becomes "abandoned". Usually, no one bothers about abandoned games but the retro gaming community, that's why it's so easy to find online games developed for now defuct platforms like Commodore 64, MS DOS, Sega Genesis, Windows 3.1, Spectrum, Amstrad CPC, etc. There were so many different home computers and consoles in the 80s and the 90s that abandonware became a niche phenomenon. If you're into abandonware for DOS and Windows, take a look at Abandonware DOS.
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Mario Bros VGA
platformer / arcade
Dave Sharpless
1990 
The Lost Vikings
platformer / action puzzle
Interplay Productions
1993 
Dark Horizons Lore: Invasion
simulation / first-person shooter
GarageGames
2005 
Daily Sport Football Strip
strategy
Smoking Gun Productions
2000NSFW 
Railroad Tycoon
simulation / business simulation
MicroProse Software
1990 
Castle Fantasia
adventure / visual novel
Studio e.go!
1998NSFW 
Super ZZT
action / action puzzle
Epic MegaGames
1992 
ZZT
action / action puzzle
Epic MegaGames
1991 
Strip Poker 2
card game
Artworx Software
1988NSFW 
Dreamweb
adventure / point & click
Creative Reality
1994 
4Play
strategy / board game
-
1995NSFW 
Atic Atac remake
action / action-adventure
RetroSpec
2006 
Castlevania: Haunted Castle 2
platformer / metroidvania
Migami
1998 
Happy Land Adventures: X-mas Edition
platformer
Free Lunch Design
2000 
King of Chicago
adventure / action-adventure
Cinemaware
1987 
Knight Lore remake
adventure / action-adventure
RetroSpec
2010 
Strip Poker Deluxe
card game
Eagle Rock Enterprises
1994NSFW 
TV Sports: Baseball
simulation
Cinemaware
1990 
Captain Comic 2: Fractured Reality
platformer / metroidvania
Michael A. Denio
1990 
It Came from the Desert
adventure / action-adventure
Cinemaware
1990 
PC*Bert
action / arcade
JROK
1997