I have personally met a few people with such good general knowledge that I was almost convinced they were aliens :-)
Seriously, though, one such example was a radiologist (now deceased) who had an excellent knowledge of all these subjects:
1) All things medical (no surprise there)
2) All things related to military/ballistics/intelligence
4) Politics (all the major countries)
5) History
6) Geography
7) Any new scientific discovery or research

Literature, Entertainment
9) The basic customs and traditions of various peoples and their religions
10) Crafts and hobbies, including remote racing vehicles, motor mechanics, printing methods, design
There was once a very tough national quiz (about 500 questions, the answers to which you had to insert into a crossword) and this dude just sat there in his chair and we read the questions and he told us the answers, he didn't need any clues from the completed sections of the crossword. The only areas where he was weak were computers, pop/modern music and anything to do with TV soapies. I told him to go on millionaire, but he just grinned. I found out when he died in 1999 that he was in fact already a millionaire and he designated alot of his money to set up a clinic in Johannesburg.
That dude was just brilliant.