It's all thanks to the SDL libraries and to the OS4 SDK environment, which makes porting software from Linux quite easier. And, as you can see, Spot is lucky enough to have more free time than me for the ports

We usually hunt the web for possible candidates, using Google and using as keywords "SDL", "game", "open source" and so on. There are also some news portals which publish news on open-source gaming. Last but not least, some ports are requested by other users (e.g. BlockOut).
Varthall