Small suggestion: I looked at a random sample - the basic dates exercise (http://pgexercises.com/questions/basic/date.html) and had a bit of trouble finding the name of the table / schema description generally. It was not obviously that this is a pop-out under "Help".
Yes, I had that feedback on proggit too - I think I'll try making a mini-tutorial that points out a couple of the more important/less obvious UI elements the first time you load one of the exercises pages.
