LouiSe has made a lot releases of open source games and demos. For some of them you need 3D graphic drivers (Voodoo graphic card atm. only, with MorphOS1.5 also with Radeon graphic cards).
You also can find more than 200 more precompiled MorphOS packages!
Screenshots and a quick list can be found here.
Download the stuff from his site at: http://louise.amiga.hu