Why is the number 666 considered evil? People always make jokes about the number 666 and how it's evil and such. What is the basis of this?
It is the number of the Beast in Revelation:


  Revelation 13:18 (ESV)
   18  This calls for wisdom: let the one who has understanding calculate the number of the beast, for it is the number of a man, and his number is 666.   


Its significance is often claimed to be that the Emperor Nero, during whose reign Revelation was written, has a name with the numerical value 666. Citing Wikipedia:


  The Greek spelling, "Nerōn Kaisar", transliterates into Aramaic as "נרון קסר", nrwn qsr. The Aramaic spelling is attested in a scroll from Murabba'at dated to "the second year of emperor Nero." Adding the corresponding values yields 666.


Personally, I'm not really satisfied with this explanation at all and am waiting for other answers.