I found this on Joystiq. All credit goes to them for the find. :)

It's no secret that the hearts of Joystiq are almost exclusively the property of Portal. Sure, we have dalliances with other games, but we always know on which side our digi-bread is buttered. So it's a comfort any time we hear we're in good company, like today when we learned that Bethesda level designer Daryl Brigner had cranked out his own custom map for the game called "Ren Test 2." You can download the files here and see the download instructions here.

After spending a few minutes with the map, we can report that it is pretty difficult. It's also pretty brilliant, with a few mechanics not used in Valve's game that we don't even want to ruin for you here. If you're a Portal junkie who's hard up for a fix, you need to download this now.

I'll be messing with this map tonight! Enjoy everyone!