Bunpro

There are plenty of websites for learning Hiragami, Katakana, and Kanji. But websites that teach you Japanese grammar, with proper exercises and lessons? Not that many.

Until a group of people made Bunpro.
Being a lifetime subscriber of Wanikani (for learning Kanji), I can tell they've borrowed some nice ideas from that website, not only parts of the navigation, but mainly SRS (Spaced Repetition) to make you review your lessons regularly.
How does SRS work? If you have no problem with a particular lesson and you never fail a review, the reviews will be asked less frequently, until Bunpro considers that you've learned it for good. On the contrary, the more you fail a lesson, the less time will pass before the next review for that lesson.

The website is also designed around JLPT levels, which are official levels of proficiency in Japanese for foreigners. The levels range from JLPT N5 to N1, N1 being that you are fluent enough to understand the language in various circumstances, such as work.
The study is based on the JLPT levels and provides a lesson for each grammar point that needs to be learned to successfully pass the test. For the moment, N5 to N2 are covered, and N1 is incoming. As an example, there are 93 lessons alone just for the JLPT N5.

As for the lessons, the content is basically links to existing free online resources. My guess is that this is something that will be gradually replaced by their own content. It works fine though as there are multiple resources for each lesson. The examples, the exercises and the reviews were created by the Bunpro team however.

Finally, the Bunpro team is transparent with the users: the website is only 60% done at the moment. It is possible to get a lifetime subscription for only $90, which is really not bad considering the price of the famous Genki books for learning Japanese grammar.
Disclosure: I've been using it sparingly for a few months while it was in beta, but decided it's time I seriously work on the grammar, so I bought the lifetime subscription today :-) Other than that, I am not part of the team, and have nothing to do with them.