Other
Chemistry Guide: Structural Isomers
This site from Chemistry Guide defines a wide variety of isomers. Provides graphics for each. Positional and functional group isomers included.
Wolfram Research
Wolfram Science World: Structural Isomer
This site from ScienceWorld provides a brief but factual definition on a structural isomer. Includes a link for additional information, and the definition has a scientific style to it.