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.