Ducksters
Ducksters: Kids Science: Solid, Liquid, Gas
Kid's learn about the science of states of matter. Solids, liquids, gases, and even plasma.
Other
Elementary Science: Matter
Colorful site provides a brief summary as well as engaging pictures of the three states of matter: solid, liquid and gas.
Other
King's Centre for Visualization in Science: Scientific Models
A series of interactive applets use scientific models to teach some basic concepts of matter, including states of matter and physical and chemical changes.