Wasabi is a Japanese language instruction company located in Shibuya, Tokyo, Japan. While they offer paid Japanese lessons and instruction, their website also has a grammar guide with explanations on a number of useful Japanese grammar points. Compared to other online grammar reference guides, Wasabi has a strong tendency to combine multiple grammar points that have similar meanings, functions, or uses so that you can learn about them in comparisons. This can be seen in pages such as "Parallel Markers: と, や, か, and とか" or "Numbers and Amounts: も, だけ, しか, ばかり, and すぎる".
The explanations are concise and come with some illustrations to demonstrate usage. However, the description of each grammar point tends to be very brief, and there are usually only a few example sentences. Additionally, some explanations may not always communicate the nuance behind a pattern. While it can be a great resource as a quick reference, if you are looking for more in-depth explanations and examples, you may want to consult other grammar guides as well.
Lessons are organized by three levels: "Basic Grammar", "Essential Grammar", and "Advanced Grammar". However, some of the grammar point categorizations might be debated. For instance, a lesson entitled, "How to Express Doubts: かな, かしら, だろうか, and ではないか" is found in the "Advanced Grammar" section when most textbooks would teach such points at an intermediate level.
Additionally, this grammar guide does not contain as many entries as other popular online grammar references. Still, it does a good job of explaining many common patterns; I just wouldn't use it as a textbook replacement.