How do you spell that?

For many people words with "ie" or "ei" in them are difficult. It´s achieve, not acheive. But it´s atheist, not athiest. Saying them out loud might help.

believe, not beleive
foreign, not foriegn
seize, not sieze

Two words I find difficult are height and width, for obvious reasons.
Other commonly misspelled English words:

accommodation
address
awesome
beautiful
because
February
necessary
separate
whether
which

I've stolen borrowed that list from http://www.passporttoenglish.com/Intermediate-English/Lesson16/Spelling.html

To make things even more difficult there are some differences between american and british english, one being words like color(american)/colour(british) and flavor(american)/flavour(british).

American: harbor, honor, humor, labor, neighbor, rumor
British: harbour, honour, humour, labour, neighbour, rumour

Other sources:
http://en.wikipedia.org/wiki/Commonly_misspelled_English_words
http://en.wikipedia.org/wiki/American_and_British_English_spelling_differences