Encyclopedia of Educational Technology
This seems to be a pretty comprehensive site. According to the home page:
The Encyclopedia of Educational Technology (EET) is a collection of short multimedia articles on a variety of topics related to the fields of instructional design and education and training. The primary audiences for the EET are students and novice to intermediate practitioners in these fields, who need a brief overview as a starting point to further research on specific topics. Authors are graduate students, professors, and others who contribute voluntarily. Articles are short and use multimedia to enrich learning rather than merely decorate the pages.
The table of contents seems pretty comprehensive, and there are some videos etc.