McGraw Hill
Glencoe: Writer's Choice: Grade 6 Proofreading Practice
This site offers two proofreading exercises in which students proofread for errors in spelling, capitalization, punctuation, and usage, and then retype passages correctly. When finished they click Answers for feedback.
University of Wisconsin
University of Wisconsin: Writing Center: How to Proofread
Concise instruction on how to get the best results from your proofreading. Suggestions are divided into "Before You Proofread," "When You Proofread," and "When You Want to Learn More." The latter includes links to handouts on writing.