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Starfall
Starfall: The Lone Vowels
Grammar rules taught through song. A familiar tune helps to teach the E and O at the end of the word that make a long vowel sound. There are opportunities to demonstrate understanding by identifying words that that follow the rule.
Other
Reading From Scratch: Spelling Rules
The "Reading from Scratch" site provides a list of spelling rules with examples.
McGraw Hill
Glencoe: Writer's Choice: Grade 6: Spelling Practice
"Spelling Practice" offers links to five spelling lessons: long vowel spellings, perplexing words, spelling the "seed" sound, the final "y", and the prefix "ad-". Each lesson provides links to practice activities.