Sample expository/explanatory essays can be found at About.com on the Secondary Education collection of pages under Curriculum Areas: Language Arts: Effective Writing Instruction: Effective Essays, Paragraphs, and Papers. The Grammar and Composition collection of pages at About.com has samples of expository essays in its Glossary of Grammatical and Rhetorical Terms.
Explanatory essays fall under the traditional discourse mode known as exposition. Exposition itself can be subdivided into different modes of development, including analogy, analysis, cause and effect, classification, comparison, definition, examples and process analysis. In addition to About.com, Academic Help has examples of expository essays in these various modes of development.