This is my go-to site for seeing techniques I'm having trouble with and learning new ones. The diagrams are amazing and Techknitter is very responsive to questions. One of my favourite sites!
Excellent library of techniques, clear and well diagrammed instructions, and details explaining the way knitted fabric works, enabling the knitter to work out further solutions independently. An invaluable resource that should be on any knitter's bookmarks!
Techknitting describes a wide range of knitting techniques with useful diagrams and clear, simple text. My favorites are the explanations of typical mistakes and how to solve them.