There's been some controversy about the use of Japanese on this forum, since some people think this is not the appropriate place for practicing Japanese, so I thought it would be nice having a list of places were everybody agrees it's appropriate to practice.
Let's share ideas for places (either in the internet or real, physical places) were practicing Japanese at different levels.
日本語中級会話 (Japanese Only): Let's start by this very same forum. As the title states, this thread is supposed to be a place for intermediate learners.
Lang-8: By creating an account here you can practice writing in the languages you're learning, and get corrections by native speakers. At the same time, you can correct texts written by learners of your native language. There are also communities were you can talk to people with similar interests. Since it's a site directed to learners where you get feedback from native users, you can use it at any stage of your learning.
大手小町 > 発言小町: This is a Japanese forum pertaining to the Yomiuri Shinbun. This is the most similar place to the kind of forums that one is used to see in English (AFAIK). This is a forum made by Japanese people for Japanese people, so it will be most useful for learners at an intermediate or advanced level. It covers a broad range of topics, like human relationships, daily life, work, learning, etc, so it can give you some insight into life in Japan and the points of view of Japanese people about different topics.
2 channel: This is a very famous forum that covers a wildly wide range of topics. It's format sucks and it's a pain to read it as it is, but there are programs to make it more legible.
I'm sure there must be many more places like these. What's been your experience with writing Japanese online?
It's important to note that if you want to be good at speaking or writing, it's important for you to listen/read before you speak/write. Fortunately with internet there are lot's of spaces where you can do that before going to far from your comfort zone.
If people contribute with more references, we could compile them and create a list at Reviewing the Kanji's Wiki
Let's share ideas for places (either in the internet or real, physical places) were practicing Japanese at different levels.
日本語中級会話 (Japanese Only): Let's start by this very same forum. As the title states, this thread is supposed to be a place for intermediate learners.
Lang-8: By creating an account here you can practice writing in the languages you're learning, and get corrections by native speakers. At the same time, you can correct texts written by learners of your native language. There are also communities were you can talk to people with similar interests. Since it's a site directed to learners where you get feedback from native users, you can use it at any stage of your learning.
大手小町 > 発言小町: This is a Japanese forum pertaining to the Yomiuri Shinbun. This is the most similar place to the kind of forums that one is used to see in English (AFAIK). This is a forum made by Japanese people for Japanese people, so it will be most useful for learners at an intermediate or advanced level. It covers a broad range of topics, like human relationships, daily life, work, learning, etc, so it can give you some insight into life in Japan and the points of view of Japanese people about different topics.
2 channel: This is a very famous forum that covers a wildly wide range of topics. It's format sucks and it's a pain to read it as it is, but there are programs to make it more legible.
I'm sure there must be many more places like these. What's been your experience with writing Japanese online?
It's important to note that if you want to be good at speaking or writing, it's important for you to listen/read before you speak/write. Fortunately with internet there are lot's of spaces where you can do that before going to far from your comfort zone.
If people contribute with more references, we could compile them and create a list at Reviewing the Kanji's Wiki
I've made a few friends there who I chat with or exchange emails with. The problem with that is my emails/chats never get corrected for errors 
So I have to make sure that I keep on posting something in Japanese.
