The bhut jalokia, or ghost chili, has a Scoville heat measurement of 1 million units—compare to jalapenos, which top out around 8000 units. Prior to 2003, it was just a myth, lacking even photographic evidence. Now, it's in the Guinness Book and will soon be the key ingredient in tear-gas hand grenades used by the Indian military. Apparently, it also has an appealing citrus flavor, followed by a horrible burning sensation.
For a quick look at what jolokia hath wrought, fast forward to about six minutes into this video. There's much less running about screaming than I would have guessed, but the man ends up looking very physically ill—pale, sweaty and generally like he just came down with a bad case of flu.