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.
Other
Sta: Punctuation and Capitalization Rules
Use this handy guide for a quick reference to punctuation and capitalization rules. Includes rules for end marks, apostrophes, capitalization, commas, quotation marks, and many more!