Web 2.0 (that I find useful)

• Wikispaces : free wiki hosting
• Backpack : free file, image, calendaring, todo list hosting (pages can be marked private)
  
• Farecast : airline fare predictor
  
• Zillow : real estate pricing predictor
  
• Digg : social bookmarking/linking site
  
• Google Calendar : online calendaring
• Del.icio.us : social bookmarking