The Mighty Apostrophe
Nice PowerPoint presentation on using the great one to create possessives, contractions and plurals. Use the apostrophe well and it will be there whenever you need it.

The Apostrophe
This piece explains the apostrophe and its uses.

Apostrophe Exercises

Exercise 1: Using the Apostrophe
What are the functions of the apostrophe? They’re explained in this creative, interactive tutorial.

Exercise 2: Apostrophe Review 
After completing Exercise 1, an opportunity to consolidate your apostrophe knowledge.
 
Exercise on Plurals and Possessives

Exercise on Plurals and Count/Non-Count Nouns

Exercise on Possessives and Irregular Plurals

More on Apostrophes

The Apostrophe Protection Society aims to preserve the correct use of this much abused punctuation mark.

Apostrophe Abuse
Links and visuals illustrating an orthographic pet peeve

Bob’s Guide to the Apostrophe, You Idiots
A one page illustrated guide to the most misused punctuation mark in the English language.

Ongoing and ever-growing list of apostrophe errors found on the web
From Terribly Right

The Long Campaign to Abolish the Apostrophe
Richard's Grammar & Composition Blog summarizes the position of those who wish to smite the apostrophe.
 

Possessive Forms
This piece deals with all things possessive: forming possessives, possessives and gerunds. Possessives vs. adjectival labels, possessives of plurals, compound possessives, possessives and compound constructions.

Exercise on Plurals and Possessives

Exercise on Plurals and Count/Non-Count Nouns

Exercise on Possessives and Irregular Plurals

Usage & Grammar

Subject-Verb Agreement

Pronoun References

Apostrophes

Commas

Colons & Semi-Colons

Spelling

Words that Sound Alike

Capitalization