I don't know about similar, but some of the same themes and tone were present in Texhnolyze (go figure, some of the same people), which you've probably seen. The same gritty distopia, a search for meaning, the world as a mystery and a riddle which is only really understood at the end (wait thats a lot of stuff). Ergo had a better sense of a journey taken, Tex didn't quite manage the same sense of motion, both externaly and emotionally, except in an internal manner. If anyone does finder something of the same nature as Ergo I'd love to know, Ergo was one of the better anime series that I've seen.
