Ducksters
Ducksters: Kids Science: Solid, Liquid, Gas
Kid's learn about the science of states of matter. Solids, liquids, gases, and even plasma.
PBS
Pbs Learning Media: Mystery Mud: Exploring Changes in States of Matter
Join a group of middle-school students on a visit to a laboratory at the Massachusetts Institute of Technology, where they experiment with "mystery mud" and learn about the relationships between magnetism, particle motion, and changes in...
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.