A lot; he was widely recognized as a genius in his own time. Sadly, his helicopter wouldn't have worked, and he never sketched an ICE.
helicopter prototype was off (just needed different shape propeller, additional stabilizing propeller and proper weight distribution), but it was on a correct development path. parachute design was correct as was demonstrated in 2000. (http://4.bp.blogspot.com/-bgE0_zA9lEU/U29iF1bk9QI/AAAAAAAAEu...)
what if tesla's highly speculated work is even 50% close?
we had thousands of electric cars manufactured and sold in USA 100 years ago (stop and think about it for a second: https://s-media-cache-ak0.pinimg.com/originals/80/34/c5/8034...). wireless transmission of electricity/energy could put us on a completely different path as a humanity (as well as completely change political and business landscape worldwide).
And an ENGINE!