Fan Sites

• http://developer.valvesoftware.com/
  Valve Developer Community
  
  
• http://www.halflifesource.com/
  Comprehensive Information on Half-Life 2.
  
  
• http://www.teamfortress2fort.com/
  A great TF2 fansite. 

• http://tf2spreadsheet.blogspot.com/

Official Links

• http://www.half-life2.com/
  Official Half-Life 2 Website
  
  
• http://www.teamfortress.com/
  Official Website for Team Fortress Classic (TFC) and Team Fortress 2
  
  
• http://www.valvesoftware.com
  Valve Software's official website
  
  
• http://www.steamgames.com
  Official website of Valve Software's broad band distribution software Steam.