English Plus+
Grammar Slammer Glossary: Prepositions
This site offers a basic definition of prepositions, along with a chart of the most common ones. Some of the prepositions listed serve as a hypertext link to more detailed information on that specific word.
University of Victoria (Canada)
Elc Study Zone: Basic Prepositions
This site explains the various uses for "in," "on," and "at" in phrases referring to transportation, time, and communications (TV, radio, etc.). Also links to a practice exercise.
University of Calgary
University of Calgary: Parts of Speech: Preposition & Object of Preposition
Offers a good explanation of prepositions and the object of prepositions, along with a list of common prepositions and several specific preposition tips.