Mac Source Ports is a wonderful website started by @TomKidd and @Schnapple.
It is a collection of open-source ports to let you play classic games with modern Macs. A few great examples are:
- Fallout 2
- Diablo
- Theme Hospital
- Quake and other id tech games
And you will find many more on the website.
To run those classic games, you will need to have their original data files. You can get them from your old CDs if you still have them, or buy them digitally from GOG.