While trying to learn Japanese, I've encountered a few different sites offering information and practices for people learning Japanese. In this article I'll describe some of them, say what they offer and give my opinion about them.
An information site made by the people from the popular blog Tofugu. This site is mostly aimed towards people starting off with Japanese. The site gives some general advice for when studying a language, and slowly introduces new components. The site is easy to follow and has a lot of explanations and examples. However it only covers a small part of the grammar.
This site relies on flashcards using Anki for practices and doesn't offer any other way of practicing other than using their other site: WaniKani. TextFugu is only free for the first chapter.
This site (which is suprisely still in closed beta) is also created by Tofugu. This site is aimed towards learning Particles, Kanji and Vocab and offers an interactive way to learn and review them (SRS method). The site uses funny/weird written mnemonics to quickly learn new items. The site does feel like an one-trick pony, you will get good at recognizing Kanji but you won't learn anything about grammar. Also some explanations given are vague ("Usually this means.."). It does offer a good community and the amount of content is big.
The site itself is free for just the first levels, which is enough to give you a decent feel for the website.
Known for their audio podcasts. It has a lot of good content hidden in a somewhat messy site. The audio files are of good quality, but they are uploaded in collections which sometimes overlap. It also offers a transcript and a small review practice for each audio lesson. Also included is a library with grammar explanations (in advanced English and with limited examples). It offers lists with introduction level vocab. The site has multiple membership levels including a free level with just a few things available. A little fun feature is The Word of the Day which can send you an email with a vocab every day.
A big warning for this site: The site really likes sending mail. As soon as you subscribe, you will get spammed with discounts and offers for their non-free membership programs (even if you are already a member). Also: Never get a membership full price, there is always big discount.
My own site, but also a site I personally use for my own study so I have to include it in this list as well. The main site offers information about a wide selection of common Japanese Grammar, Vocab and Kanji. It also has a list of example sentences.
The learn center (free for 2 weeks) offers interactive practices to learn and remember Grammar, Vocab, Kanji and Sentences. After the introduction lessons and practices, you get freedom to select sentences you want to learn, the site will make sure you learn all the grammar and vocab in those sentences. It also features regularly reviews (SRS method). The site is able to run offline as well, including on your mobile.
Probably the most complete site you will find online about the Japanese grammar. It also has a few small exercises and it is free. The content itself is fast paced but it does include examples. There is also an app available.
A good free overview of the Kanji, Vocab and Grammar required for each JLPT level, including audio files. When learning for the Japanese-Language Proficiency Test this site can be used to review that you actually learned every bit you have to know before doing the test.
This is a site recommended to me, and after trying it I had to add it to this list. On this free site you can quiz yourself on Kanji and Vocab knowledge. You will score points by answering questions right. Great for people who need some extra motivation. There is also an iOS app available.
This was just a small list of resources I'm familiar with, but there are of course many more sites aimed towards people learning Japanese. Just poke around on Google, look up videos on YouTube and avoid sites which claim you'll learn Japanese with no effort at all. If you also have an interesting site you want me to take a look at, feel free to contact me.