You've got to respect a little cross-developer love. Bethesda level designer Daryl Brigner has taken it upon himself to create a new, and what sounds like difficuly, Portal level.
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.
You can download the level here.