The Autobahn is one obvious answer: though it was not begun under Hitler, a massive expansion in the 1930s (partly as a public-works program for employment and also as twin propaganda/pork for local voters: see this recent working paper, which suggests opposition to Nazis fell perhaps 50% faster in areas connected to the network compared to those that weren't). The short-term economic effects appear not to have been substantial (see paper above for citations), but arguably the legacy of a dense modern road network was an important part of West Germany's post-WWII Wirtschaftswunder.

I would guess there might be more important answers (perhaps rocket technology, if you wanted to count that as a positive), but I can only speak to the economic history on this.