Web of Science

• Web cyclopedia
  • Wikipedia -- I find their website slow often times
    
  • Wolfram ScienceWorld
    
  • Eric Weisstein's World of Mathematics
    
  • Mathematical Giants -- excellent!

On-line Dictionaries

• UC San Diego Webster Dictionary -- comes in handy especially if English is not your native lagnuage :)
• Latin Words and Phrases -- ugly background, but has a couple of hundred entries

Music

• AMG All Music Guide
• Real Synthetic Audio -- great Canadian site featuring electronic/industrial music