Main links:

• ObjectVideo - the company that brought you Virtual Video
• Valve software - the company that brought you Half-Life 2
• Half Life 2 - official page
• Steam - content delivery software

Half-Life 2 modding resources:

• The Valve Developer Community wiki - official developer resouce pages
• Planet Half Life - news and modding resource
• Half-Life Fallout - news and modding resource

Articles:

• Planet Descent Article - comparing Half-Life 2 performance on different graphics cards.

Third-party Tools:

• Blender - a popular, free 3D modeling tool.
• VirtualDub - a popular, free video manipulation tool.
• GraphEdit - a free graphical utility for debugging DirectShow filter graphs.
• GIMP - GNU Image Manipulation Program: a popular, free image manipulation tool.
• MSDN DirectShow documentation