Though I completed Portal2 what seems like an age ago now, I still have good memories of the incredible quality the Valve team was able to squish into every corner of the game. The CGSociety article covering Portal2 design shows off some of the depth behind the scenes and gifts us with some of the concept art. More at the COMBINE OVERWIKI.
[Portal 2 concept art via the Combine OverWiki]
The Portal2 Music Video Contest had a very nifty set of entries, here are the 2 winners and the coolest lego one :-)
This concept of a pair of portal rings by yuumei is gorgeous:
[Yuumei's portal engagement rings via her DeviantArt page]