Other
Michigan State University: Isomers
This in depth article on isomers has sections that range from basic information to stereoisomers. A section with diagrams is also included.
Clackamas Community College
Clackamas Community College: Alkenes
Clackamas Community College provides a great resource on alkenes. This site teaches you about the nomenclature and structure of alkenes. It also discusses the two types of isomers alkenes can form and addition reactions.
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.