Links

(I have copied the headlines of all pages next to each link, i.e. naming and content are not mine.)

Rebecca Treiman 
http://spell.psychology.wayne.edu/

Brett Kessler 
http://spell.psychology.wayne.edu/~bkessler/

English is Tough Stuff
http://www.yourdictionary.com/library/tough.html

Common Errors in English (have a look at the links 'List of Errors' and 'Commonly Made Suggestions' at bottom of page) 
http://www.wsu.edu:8080/~brians/errors/index.html

The Problem of English Spelling 
http://www.ncf.carleton.ca/~aa735/eng_sp.html

Online Writing Lab 
http://owl.english.purdue.edu/handouts/grammar/

(American) English Spelling 
http://www.iss.stthomas.edu/studyguides/spelling.htm

Notes on American English 
http://www.scit.wlv.ac.uk/~jphb/american.html

Absolutely Ridiculous English Spelling 
http://www.say-it-in-english.com/SpellHome.html

28 Rules for English Spelling 
http://www.riggsinst.org/28rules.htm

Spelling differences between American and British English 
http://www.gsu.edu/~wwwesl/egw/jones/differences.htm

Language Fun Stuff & Language Games 
http://www.yourdictionary.com/fun.html