There are, unfortunately, some bugs in the quizzes. When I took the Already/Yet quiz (Intermediate) I was surprised at a couple of the answers I had chosen. I thought maybe I had somehow goofed, but it happened again on the Quite/Rather (Advanced) quiz (which seems to be a test of BE). Just to be sure, I clicked on Reset then took the quiz again and wrote down my answers. Sure enough, on question 3 I picked 3, but the computer told me I picked 2.
A question. In the quiz on British spelling you have two questions about organize. Did you mean to do that? (I got one wrong and one right. ;-) ) (Overall, I got eight out of ten, which I think is not bad.)
Overall, it looks pretty good. It should prove quite helpful to ESL learners. Do you want me to tell you about any typos I find?