English Plus+
Plural Possessives
This site discusses the correct and incorrect usage of apostrophes to show possession with plural nouns.
Capital Community College Foundation
Guide to Grammar and Writing: The Apostrophe
What is an apostrophe? Where are apostrophes found in writing? Use this website to learn more about these crazy little punctuation marks.
Other
Illinois State U.: Punctuation Made Simple Apostrophes
What is an apostrophe and how is it used properly in punctuation? "Punctuation Made Simple," gives some concrete examples for apostrophe use.