University of Ottawa (Canada)
University of Ottawa: End Punctuation
Resourceful site which includes information about periods, question marks, and exclamation marks. Also provides examples and addresses abbreviation rules for periods.
University of Calgary
Univ. Of Calgary: Periods, Question Marks, Exclamations
This excellent resource provides information about rules, uses, and misuses of these three end marks (period, question mark, exclamation mark). Also, includes examples and short quizzes at the end of each section.