Tae Kim’s Guide to Japanese Grammar is one of the best, general use Japanese grammar guides I’ve seen.
The guide caters to learners of all different levels, from the very new beginners, to the advanced learners. I’ve been reading the guide myself for some time and I’ve found it to be very informative. Practice exercises and reviews are available too, to help you reinforce your learning.
The best part? The guide is free (unless you want the physical book), and available in all sorts of formats. You can view the guide from the website, download the pdf version, read from your phone using the iOS app or Android app, or for traditionalist, buy the physical book from Amazon.
There’s a slight downside though. Unlike many guidebooks that have split their material by JLPT levels, this guide does not make such a differentiation, and as such, might not be ideal for those of you who are cramming in bid to past that particular JLPT level.
Still, with so many available formats, there is no excuse for you not to check it out!