It's worth noting that most of the major Allied technologies we associate with the European war were secret (e.g. the code-breaking computers) or subtle. The most important two were probably radar and the proximity fuze. Neither of them have a "Wunderwaffen" quality to them. The Germans had their own proximity fuzes (though they didn't deploy them as heavily), they had their own radar (though they didn't develop them as well or integrate them as successfully into their total defensive and offensive plans). The Allies also used napalm to deadly effect — again, not much of a wonder weapon, just a good improvement on existing techniques (incendiary bombs). I am wracking my brain for a good Allied technology that mattered during World War II that would have actually been a significant propaganda tool, or made a big morale splash, and just not coming up with any. The German technologies were superficially impressive but ended up being not very effective militarily in the forms developable for the war. The Nazis led the "super technology propaganda" war during the war itself even though the Allied technologies, while superficially more banal, were much, much more useful for winning the actual war.

The atomic bomb, of course, would qualify as a wonder weapon but, of course, it was not developed in time for use during the European campaign. The physicists who were working on fission work for the Germans were justifiably shocked that the Americans had managed to pull that one off, when they heard about it after Hiroshima.