A lot of ESL students have difficulties with spelling. The differences between American and British English are no help. I tell my students that as long as the word is spelled correctly in one of the two languages, it doesn't matter. Normally they are learning it for work and, let's be honest, most people outside of the States and Great Britain don't speak English as their native language and may not know the differences themselves.
Here's a short quiz students can take to check some commonly misspelled words. http://a4esl.org/q/j/ck/mc-spelling.html On a similar site you can find a lot of games to play online, which would be fun for young and old alike. http://www.manythings.org/cts/
Another question which arises in this area is, of course, should I correct students' mistakes? Well, I don't always do this, especially in very low levels. My reasoning is this; when I was learning German I often felt completely overwhelmed simply with pronunciation, so trying to spell the words and get everything grammatically correct was simply out of the question. It took me a while before I could do everything simultaneously. So, I keep this in mind when teaching beginners. If the word could be confused (i.e. their, they're, there) I correct them and we normally have a good laugh about how messed up the English language is. You can find more on this topic here http://esl.fis.edu/teachers/support/faq1.htm#2. There is also a link for students to practice spelling by playing games on the site.
So here's a little note to students. You can find rules and help with spelling here http://www.really-learn-english.com/how-to-improve-your-spelling.html
I'm still looking for a pdf to post for students with all of the rules and as soon as I find something I'll definitely post it.