shiva/ston3d follows similar goals: real time shadow mapping, post processing and easy to use shaders, cross-platform, in-browser gaming. the shadow renderer is not that advanced as unity's atm (only 1 shadow per model, point lights cannot throw dynamic shadows yet) but i think it will have a bright future as it's announced for the 1.7 release. http://www.stonetrip.com and it's available for windows, too. looking at the editors, i must say that they don't differ too much, they even got both the play/pause/reload -viewport buttons. unity-like terrain features are on the way.

about getting mac os x on your pc, *solely* for educational purposes, of course: http://wiki.osx86project.org/wiki/index.php/Vmware_how_to