Spelling-- children with ADHD are often phonetic ( i.e. "poor") spellers. They have difficulty using an ordinary dictionary, because they can not look up the word the way it sounds. Small paperback dictionaries , such as the " Poor Speller's Dictionary" exist, and are often very useful. They contain words spelled phonetically along with their correct spellings. Just remember to lose the cover before the child takes it to school. Who wants to carry a book that says " I can't spell" on the cover?
Computers-- Word processors and spelling checkers can help, but they only decrease the work on rewrites. Simple solutions include the use of macros in word processor ( common phrases are typed with a single key stroke) and having someone type from a taped dictation.
A high tech solution is to use one of the newer voice typing programs. Voice Direct by IMSI is very inexpensive and quite good in voice recognition. A problem with the newest voice typing programs is that they do not "keep up", processing words to the screen somewhat after they are spoken. This can be disconcerting for the ADHD individual. Programs that...require...discrete....words...seem...to...work...better.