Spelling it Right - Roger Smith
This site has lots of advice, many worksheets and lists to help with spelling. It is by a British teacher, so be aware of some spelling differences. (For examples of some of the main differences, see English Club - English and American English Spelling).
Interesting Things for ESL Students - Spelling Practice
Games and Quizzes
Help with Spelling Problems for ESL Classes - About.com
Catch the Spelling - Manythings.org
Lots of spelling games at three levels: easy, medium, and difficult