Teachers can create an account for school. Provides practice and quizzes. Probably best for elementary or sped. I also like SpellingTube which has videos created by students (UK accent) to reinforce spelling.
This is way cooler than I thought it would be! You input some vocab or spelling words and it creates tests or teaches the words including spelling, and using them in a sentence.
A superb resource where teachers can sign in and input spelling lists for pupils to learn by playing games. Give pupils the link and they don't need to sign in to use it. Site only recognises US spelling when generating example sentences, but you can input your own easily. Free option should be enough for most users, but 'paid for' option is available.
http://ictmagic.wikispaces.com/English