Ghotit is a company founded by people with dyslexia and they have improved upon their spell checker and word prediction product for both Mac OS and Windows.
By having an expanded set of rules that includes a wider range of spelling errors, Ghotit appears to work better for those with severe spelling errors than say, the built in spell-checker of Word. I’ve tried it out with a couple of students and it found more words than other products. It has nice text to speech capacity so that when choosing a word for either spell check or its word prediction capacity, a user can make sure they’re selecting the word they want.