Other
Project l.a.: Possessive Nouns
Designed for elementary students, this Internet viewable slide show explains how to form possessives, then provides practice sentences. At the end are links to an online practice quiz and a printable worksheet.
Bartleby
Bartleby.com: William Strunk on the Possessive Singular
William Strunk Jr.'s "Elements of Style" on the creation of possessive singular nouns. Provided by Bartleby.com
Other
K12 Reader: Possessive Nouns
Take a look at examples of possessive nouns, both singular and plural.