We have Tesla to thank for the fact that we almost universally use commercial AC power in our homes and industries. This is a legacy of huge impact: had we gone the Edison/DC route a lot of things about the day-to-day technology we use would be very different.
As you mention, he also invented the AC motor. Pretty much every plug-in motorized appliance you use -- clocks, can openers, refrigerators, clothes washers & driers, garbage disposals, electric drills and saws, etc., etc. -- use some variation of Tesla's invention.
It was Tesla that very likely invented, or at least first demonstrated the real-world feasibility of radio. Marconi's famous demonstration in 1901 was achieved using devices and components covered by 17 different Tesla patents, going back as far as 7 years. Tesla sued Marconi, and in 1943 (after Tesla's death) the case had worked its way to the Supreme Court, which found in favor of Tesla.
Tesla was a pioneer in concepts of heterodyning, RF feedback, high-frequency transformers, and ELF transmission. Experiments he performed in 1917 laid the groundwork for the development of radar.
He was not without his woo-woo side -- I'd call him an eccentric (rather than "unstable") genius--he brought genius to every project he undertook, even when the project was nutty. He was a horribly impractical businessman, and repeatedly had his inventions and patents ripped-off by people who made their own fortunes on them. Usually he let this go by; the Marconi case was unusual. He believed in extraterrestrial civilizations and spent time developing means to try to contact them. He was obsessive/compulsive, inordinately fond of pigeons, and had a horror of germs.
He came to believe that high-frequency / high-power AC transmission was the solution to a vast array of practical problems from defense (his "death ray") to free electricity for home and industry.
But come to that, there are plenty of other highly intelligent and accomplished scientists and engineers who developed a fascination with one or another woo idea in their lives. Think of Nobel prize-winning Linus Pauling, and his almost fanatical advocacy of vitamin C, or Shockley, and his obsession with race, intelligence, and eugenics.
All in all, Tesla is either unknown or under-appreciated by much of the society he helped to make possible.
This is not a bad place to start, if you want to learn more about this fascinating man:
http://www.amazon.com/Prodigal-Geni...=sr_1_4?ie=UTF8&s=books&qid=1266453416&sr=1-4
A word of warning, though: there are a lot of "Tesla" books out there; I recommend avoiding any that have the word "Lost," "Hidden," "Secret," or "Occult" in the title. As
didalb said, there is a lot of nonsense about Tesla in circulation.
