Wikipedia has a Simple English version; it's primarily intended for users for whom English is a second language, but it's also for children and those who want to learn at a more leisurely pace - it's effectively a Newsroundised version of Wikipedia. Amusingly, it contains an article on M-theory.
Unfortunately, the article is currently marked Unsimple. I've simplified the section on string theory, under the motivation that if M-theory (and superstring theory and friends) can be made “simple”, anything can. The rest of the article, however, still needs more gigawatts.
If you've got five minutes to spare, try translating an M-theory article into Simple English - it makes a fun Sunday afternoon activity.
...And, as is to be expected from any project worth its salt, there's also a (somewhat more modest) tlhIngan Hol wIqIpe'DIya.