Imabi is a Japanese grammar guide site with over 400 entries covering anything from specific grammar patterns, to words and history. Some examples of entries include "The Particle Wo 1", "Giving Verbs, "The Causative-Passive, "やはり and さすが, and "自発動詞". All lessons are broken down into levels and categories: Beginner 1 & 2, Intermediate 1 & 2, Advanced 1 & 2, Veteran 1 & 2, Classical Japanese, and Okinawan Japanese. This particular ordering of grammar patterns allows the site to be potentially used as a textbook replacement.
Imabi's grammar explanations often include information that's useful for Japanese learners, but that may not be found in other online grammar resources. For example, their page on sentence-ending particles よ and ね covers information about how the difference in intonation can result in different nuances. You can also find notes about idiomatic expressions that use the grammar point in question at the bottom of many of their grammar pages.
While the writer of this site is not a native Japanese speaker, their lessons and explanations are excellent. According to the main page, they have received a lot of input from native speakers.
The site design could be described as simple; the lessons are very organized but some may find the lesson layout and formatting to be crude and inconsistent. But besides that, the website is an excellent resource for anyone looking for a grammar reference.