The fan-run rules encyclopedia, which is actually more comprehensive than any printed rules, covering many edge-cases and multi-expansion interactions that are not covered in the printed rules.
The site also has several interactive tools including:
An excellent digital implementation of Carcassonne, featuring more expansions than any other digital version.
The official online shop for Hans im Glück. Usually the first distributor of new Carcassonne expansions, and often the sole distributor of the less common expansions.
The official publisher of Carcassonne for the North American market.