Grammar Base - a new free online grammar check service. http://www.grammarbase.com/check/.
This was new to me, so I opened it up. It provides free grammar advice online, and seems to pop up early when you are looking for grammar advice. So you type stuff in there, and it tells you: Passive. Misspelling. etc. etc. This implies, to me, that Passive is in the same class as misspelling. How is a student to know? The big sign says: Passive!