Martin Heidgegger died before completing his attempt to overturn the entire philosophical tradition. On the way, though, he did come up with a new way of thinking about “being” that has largely been incomprehensible to anyone without a properly functioning Department of Philosophy to hold their hand and guide them through specialized language, questionable translations, and impenetrable prose.
The University of California at Berkeley has made Hubert Dreyfus’s class on Heidegger available as a podcast. Professor Dreyfus has also provided some useful notes and a reading list. One of the books he recommends, William Blattner’s Being and Time, A Reader’s Guide, is particularly lucid. That particular book and Professor’s Dreyfus’ lectures compliment one another very well.
That said, the kindest thing that can be said about Heidegger as a human being is that he was deeply flawed man. Rüdiger Safranski’s biography is also a good introduction to Heidegger while addressing his behaviour in Nazi Germany.
Another quick introduction to Heidegger is available in the Internet Encyclopedia of Philosophy.