This site discusses the misuse of the apostrophe, such as using an apostrophe in a word which is a simple plural, now known as "greengrocer's apostrophe", the contraction it/it's, who's/whose, etc. A trend toward dropping the possessive...

Learn to use apostrophes to indicate possession, or ownership -- and when not to use them.

This article focuses on teaching 2nd graders to form possessive. It provides two complete lesson plans with activities for teaching them how to form possessives.

What is an apostrophe and how is it used properly in punctuation? "Punctuation Made Simple," gives some concrete examples for apostrophe use.