Punctuation Handouts
http://owl.english.purdue.edu/handouts/index2.html
Excellent handouts and information covering the use of the comma, semicolon, colon, parenthesis, dash, quotation marks, and italics. In addition, there are on-line exercises for students to complete along with answer keys.

Guide to Writing
http://grammar.ccc.commnet.edu/grammar/
The Guide to Grammar and Writing is sponsored by the Capital Community College Foundation. Once on the main page for this site, scroll down to the middle, locate "Word and Sentence Level", and choose from a from a dropdown menu the punctuation mark for review (i.e. commas, semicolons, apostrophes).

Grammar Slammer!
http://englishplus.com/grammar/
There are explanation, rules, and examples for virtually every punctuation mark including: the period, question mark, exclamation mark, comma, semicolon, colon, quotation marks, italics, underline, dash, parenthesis, hyphen, apostrophes, ellipsis, bracket, and virgule.