I cringe when people misspell words on purpose. "Kewl" instead of "cool", "nite" instead of "night"... there's no real need to misspell these words. They aren't so much shorter that it saves time (the former might actually take longer to scrawl out, if writing by hand), and to me it just seems to butcher the English language. There are, however, times when we misspell words accidentally, due to the trickiness of silent or double letters, or similarity to other words. These three websites could be useful if you find yourself repeatedly misspelling the same words:
ESLDesk Commonly Misspelled English Words features 507 words with links to their corresponding page on a number of well-known sites: Wikipedia, Cambridge, Encarta, and Merriam-Webster among others. It also links to translations in 21 languages. It's great because of the number of other websites that it incorporates and would also make for an excellent teaching aid.
YourDictionary.com has a 100 Most Often Mispelled Misspelled Words in English page as well, with helpful hints on how to remember the correct spelling of each word. For example: "apparent: a parent need not be apparent but 'apparent' must pay the rent, so remember this word always has the rent".
AskOxford.com uses a similar style in their Commonly Misspelled Words page as the YourDictionary website. AskOxford includes related words, tips, and rules for each one to help the reader out, although their list is not nearly as long as some of the other websites.
If you aren't sure about how to spell a word, don't guess at the spelling- look it up! There are helpful databases all over the Internet for these situations. Check out the above sites to see if there are words that you have been unknowingly misspelling. When in doubt, double-check, and use the hints and memory tricks suggested at the above websites so that next time, you won't have to look it up.
What are your favourite tricks for remembering how to spell words that you find especially difficult?