When Word 97 came out I stayed with Word 6 for a long time. The reason was that I was doing my A Levels at the time and the grammar checker on Word 6 by default gave you the Flesch Reading Ease, the Flsech-Konrad Grade Level and a Readability Index. This was useful for checking whether I was using too many or too few hard words!
Google Docs & Spreadsheets has just been upgraded to include this information when you request a word count. I copied-and-pasted a recent blog post from teaching.mrbelshaw.co.uk and the results can be seen above.