These are some websites containing good expositions of relatively simple Mathematics, demystifications of complicated Mathematics and other things in similar spirit.

• Eric Rowland's Investigations in Mathematics
• Better Explained - WOW
  
• Timothy Gowers' Amazing Blog (e.g. Removing the magic from Stirling's Formula)
• Timothy Gowers' Amazing Website (e.g. Commutativity of Multiplication)
• Eric Schechter's website, which contains lots of important expository material and some discussions about constructivism and The Axiom of Choice
• A trip through Euclid
• David Joyce's Homepage, with beautiful lessons to teach basic ideas in Mathematics that seem like the way these subject could've been studied when they were "real" Mathematical topics and not "just tools" for more advanced Mathematics, as we see them today.
  
• Robin Hartshorne writes about teaching Euclid
  
• Infinite Ink
• TeX THE WORLD, a plug-in that allows you to use TeX formulas in ANY website (like gmail, google chat, forums, chat rooms, etc).
• Twiddla. A whiteboard to talk about Mathematics with people in other place while on the phone at the same time. This can be used with TeX THE WORLD to type math formulas on the board.
• The Math Circle