Some fun sites:

• The MacTutor History of Math Archives
  
• 1000 Maths problems
  
• Math Forum
  
• Cayley Quilt Maker
  
• The Prime Pages (prime number research, records and resources)
  
• The National Math Trail
  
• MMSTEC's Digital and Print Library
  
• NCTM's Illuminations
  
• MMSTEC