Dark Places or Telejano. Neither is as impressive as Tenebrae in my opinion, though they're also pretty nice in their own right.
As for whether the Xbox could handle Tenebrae, I think it could. I've run it on my 900MHz P3 with a Geforce2Go card and it's actually quite playable at 512x384 resolution, and it looks absolutely beautiful. The Xbox processor is slightly slower, but the video card is much better than mine. Needless to say, I would love to see it on the Xbox. And doesn't Tenebrae use Direct3D? That would make it easier to port, wouldn't it?
And for the record, I'd also love to see Doomsday ported. There's no question that the Xbox could handle that quite well, it is a DirectX app, and the engine really does a great job enhancing Doom, Hexen, and Heretic.
But in any case QuakeX is a lot of fun, and I'm glad to see it got released!
Edit: I believe I was wrong about Tenebrae; it seems it does use OpenGL.