Actually I know what it is ... it's a large code base, fully available, very graphical, and with reasonable performance demands. So many years ago, Vertigo ported Quake II to managed C++ to show how performant the app was and how easy it was to integrate something that is quick to do in managed code - a partially transparent bad-guy locator if I remember correctly. That was five years ago, before C++/CLI, so it was a thicket of underscores and general hard-to-read misery.
About a year ago, Greg Dolley did it to C++/CLI. And then in January of this year he did Quake III Arena.
Well, now Julien Frelat has done it in Silverlight! He was apparently inspired by someone who did it in Flash. This is Quake I, but he has plans for II. Adam Kinney has an interview and video.
What else can Quake be ported to?