Portal Stories: Mel, an upcoming mod for Portal 2, is starting to less like a mod in this new trailer, and more like a straight-up official Valve entry in the series.
OK, so we can't speak for Mel's sense of humour, but everything else about it looks shockingly professional, from the detail (and unique design) in the levels to the prototype Portal gun itself.
Note that while this is a mod, it's also an all-new game: it'll feature over 20 levels and its own story (and dialogue).