My obsession and my inspiration since I gained computer-consciousness. It taught me a lot of things, and inspired me to learn a lot more. I owe a lot of what I am today and the field that I chose for myself to work in to endless nights spent Quake-hacking.
I have played a lot of Quake, on a lot of different machines, on a lot of different platforms, over many years. No version has ever crashed, even once. To me, Quake epitomizes software perfection.