Online Spanish grammar reference
I find the grammar section of studyspanish.com:
to be a helpful grammar reference.
I've observed that many schools and websites overemphasize grammar, whereas Duolingo emphasizes translation, which helps build vocabulary and its usage. Also, in my opinion, one can't help but learn how to spell--with the proper accent--if you type the Spanish answers accurately in the exercises. Thus, I find Duolingo to be a refreshing alternative to Spanish-language learning resources that emphasize grammar too heavily.