1. Bartleby.com Great Books Online -- Encyclopedia, Dictionary, Thesaurus and hundreds more



2. Longman Dictionary



3. thefreedictionary



4. Merriam-Webster Online



5. concordancers (lextutor)



6. Roget's Thesaurus



7. The Handbook of Technical Writing - Alred




8. Books and Articles on Writing Well




9. How to Spell Like a Champ by Barrie Trinkle, Carolyn Andrews, Paige Kimble




10. Collins Cobuild English Guides 8: Spelling (Collins Cobuild English Guides) by jonathan Payne, ISBN: 0003709507