Portal is a classic puzzle game set in the mysterious Aperture Laboratories. This game is easy to run and very replayable.

Game Dev Tycoon lets you start a business making video games and makes you adapt to the market and people's interests. Great, fun game and easy to run.

Sven Co-op lets you play the original Half Life with others. Amazingly fun and should be able to run on anything made after the Commodore 64.

Gunpoint is a easy to run side scroller stealth action game set in a noire type setting. Play as Conway, who seeks to solve a murder and clear his name.

A classic game that surprisingly many people haven't played. Set in a SciFi universe where humanity is enslaved and a rebellion is brewing. A must have.

Papers Please is a interactive immigration officer simulator where you learn the story through speaking to it's characters. A Good game and easy to run.

Highly moddable, infinitely replayable and also isn't very hardware demanding. If you don't have it already, you should get it.

An old completely moddable game that's still very much alive with custom servers- Gmod runs very well on almost all computers.