Journalist and educator Mindy McAdams posted a quiz at:
http://www.sentex.net/~mmcadams/spelling.html.

The People's Almanac features a page of Commonly Misspelled Words at:
http://www.trivia-library.com/a/list-of-commonly-misspelled-words.htm.

Finally, Scripps, sponsor of the National Spelling Bee, offers a download of its Consolidated Word List. But beware: its nearly 800 pages contain more than 20,000 different words for study. You can find it at:
http://www.spellingbee.com/resources.asp.