The good people over at ValveTime.net assure their Facebook followers that they have gotten some sweet concept art of that fabled Episode 3 of Half-Life 2, all the way from 2008. Here are all of the photos attached.
Not sure what all of them are, but some seem to be new characters and possibly some new enemies or evolved forms of older enemies.
Keep in mind that ValveTime is saying this is definitely official concept art, but it is old, all the way back from 2008.
Source – ValveTime Facebook Page