Nethack - the ultimate low end system capable game
Nethack is now 36 years old, still in development, runs on windows, Linux, Apple and a multitude of other systems. You can run it in a console, remotely, if you don't have the ability to install or download to your computer. It's complex, difficult, entertaining and will soak up any free time you have.
I've played it on and off for a decade now and never made it past half way and have always learnt something new.
Been playing for over 10 years now. I love it, it’s incredible and I’m so glad it’s still around. Might have to make a go for the amulet again today, dust off the old files.