I didn't know this existed until it was referenced briefly in an unrelated article. Simple English Wikipedia:
This is the front page of the Simple English Wikipedia. Wikipedias are places where people work together to write encyclopedias in different languages. We use Simple English words and grammar here. The Simple English Wikipedia is for everyone! That includes children and adults who are learning English.
Great idea! I perused some of the articles relating to linguistics, language, and grammar. Not bad. Under Grammar, they write:
When we speak, we use the native-person's grammar, or as near as we can. When we write, we try to write with correct usage grammar. So, speaking and writing a language each have their own style.
While one may want to quibble a little with this (I don't quite understand the "as near as we can" hedge), it's actually a nice way to explain the difference between speaking and writing to, say, a 10 year old.