Usability.gov has a great overview of card sorting.
Smashing Magazine put together a beginner’s guide for card sorting.
Boxes and Arrows wrote a more detailed, “definitive guide” to card sorting.
UX Booth has a 101 to card sorting.
Measuring U breaks down card sorting into a top 10 key things to know.
Measuring User Experience has a list of card sorting tools, some free and some paid for.
Optimal Workshop sells three tools for easy card sorting.