Léon Theremin - Best known as the inventor of the weirdest ever musical instrument, this guy should almost certainly be as well-known as a hyper-intelligent inventor of loads of weird stuff as Tesla. Unfortunately he spent a large part of his very long life (1896-1993) working for Stalin whether he wanted to or not (mostly he didn't).
Back in the 1920s he was a few months too late to create the very first television, but his version, arrived at independently, was much more advanced than John Logie Baird's. Stalin immediately made it top secret because he wanted to use it for video surveillance along Russia's enormous border, though being a delicate and very expensive prototype, it was completely useless for this application.
During WWII, Theremin attempted to persuade Stalin that his talents would be best employed designing advanced long-range rocket weapons. Stalin insisted on his instead designing a super-bomber from specifications drawn up by non-scientist party officials who wanted something massively bigger than anything the Germans had just for the sake of it. He protested in vain that this monster broke the laws of aerodynamics. When it strangely failed to fly, he ended up spending a few uncomfortable years in Siberia.
However, he did design two incredibly sophisticated bugging devices, one of which was in the office of the US Ambassador in Moscow for 7 years before it was found by mistake (and then it took the CIA another 5 years to figure out how it worked).
He also invented the first drum machine, the first disco - a machine which not only produced different notes in response to the movements of a dancer, but had a psychedelic light-show synchronized to the music - and claimed in the 1930s that in the future everyone in the world would be able communicate through a fantastic network of interlinked electronic typewriters.
He also suggested some pretty far-out things, such as force-field motorway bridges, harnessing the Earth's magnetic field to turn the whole planet into a spaceship, and raising Lenin from the dead, a plan he only abandoned because the embalming process had included the removal of his brain.
If Stalin had let him develop Russian V2-type weapons, they probably wouldn't have affected the war very much, but would the Space Race have been more interesting? Or alternatively, had he stayed in America having a high old time instead of returning to Russia shortly before WWII (for patriotic reasons, to avoid paying tax, or because Stalin had him kidnapped - accounts vary), would we have had rave culture in the 1940s and a crude internet in the 1950s?
By the way, it has been seriously (?) suggested that Theremin was a "Man Who Fell To Earth"-type space alien. You can probably see why.
