The problem with extensive reading online is that it’s difficult to find material suitable for beginning readers, especially if you’re a beginning reader yourself. The good news is that there is so much material available online that I think one could put together a good course of reading material suitable for extensive reading without dealing with the expense of real books.

I’m always on the lookout for more online reading resources, so feel free to post them in the comments! I’m particularly interested in low-level resources, because those are the kinds of things that are the most useful, but the most hard to find if you’re a beginning reader. I presume that anyone who is able to read things like the texts at the Japanese Text Initiative already knows how to find them.

EhonNavi
A resource about picture books that includes ratings, reviews, previews of some books and — most relevant to extensive readers — over 800 full picture books! Click the link on the right that says 全ページ試し読み to see the available books; click a book and find the yellow icon that says the same thing to read it. You have to be a member to read them; if you haven’t signed up already you’ll be prompted when you try to access a book. You can only read a book once, and if there’s an error of some sort you should be able to re-access the book, as long as it’s within 15 minutes. The recommended OS is Microsoft Windows XP Home Service Pack 3, with Internet Explorer 8.0 or Firefox 3.6 and Flash Player 10 installed. It may not work as well with WindowsXP Service Pack 2 or Mac. I put up a walkthrough for the registration process, along with more information about the site, so even a beginning reader should be able to sign up and try reading some of these! Click here for a list of titles sorted by word count. (Generally, the lower the word count, the easier the story.)

心の絵本 (Picture Books For The Heart)
If you’re new to extensive reading, this is a great place to start, because all of the stories are nicely illustrated and short – basically as close to real picture books as I’ve found. (They are even written in all hiragana and katakana, no kanji; this is a pain for adult readers who already know and would like to practice kanji, but it is exactly what you’ll find in a real picture book.) Click here for word counts and levels for the stories. (Again, the lower the word count, the easier the story.)

デジタル絵本サイト (Digital Picture Book Site).
If you are new to extensive reading, this is another good site to start with. It has around a hundred picture books, all illustrated by children; each story is basically the equivalent of a picture book, so for a beginning reader with a good knowledge of kanji, this site could very easily replace a pile of level 2 picture books. My only problem with it is that there are a lot of compression artifacts around the text for most, though not all, of the stories, and it really bothers me, considering that actual kids books start with nice, clear, large text. If the poor quality of the text is a problem for you, start with the stories in large text — there’s about thirty of them.

福娘童話集 (Hukumusume’s Fairy Tale Collection)
With hundreds of fairy tales, short stories and fables, you’ll never be hurting for reading material as long as this site is around! Furigana is handled pretty well – it only seems to be added to a few words, which I like, because I can’t stand reading something where every third kanji is supplemented by a reading in parentheses. Many of the stories are also read aloud, so you can practice listening as well. The only drawback is that there’s limited pictures, which I think are useful in the beginning because they let the reader connect their knowledge to knowledge in the story they might not know yet and because they confirm the reader’s image of the text. Still, the stories are short enough that it shouldn’t be too bad. For those of you whose patience with fairy tales may be limited, start with the 江戸小話 (Short Stories from Edo), or make a habit of reading 今日は何の日？ (What Day Is It Today?)

The Great Chokochoko Library
A collection of reading material, sorted by level. There’s various types of material available: stories, articles and so on.

日本昔ばなし (Old Stories Of Japan)
I would recommend the other story sites over this one because it might be too tempting to rely on the English translation; there are, however, many Japanese fairy tales available here, and my impression is that they use more advanced kanji than the other two sites, so it may be worth a look.

Japanese books from the International Children’s Library
There are only a handful of stories in Japanese, but I do like to see kids’ books from the Showa era, so I’m including it anyway.

Gakken Books for Supporting Reconstruction
Gakken has made some of their books available for free in PDF form to help support the children and families still living in shelters after the 2011 Tōhoku earthquake and tsunami, as well as volunteers and nurses. The page is divided into three categories: picture books/reading material, games that can be played without special equipment and disaster nursing.

船 (The Boat)
This is, unfortunately, just one story, but if you are thinking about buying the よむよむ文庫 Japanese graded readers (or of course, just for reading practice in general), try this Level 1 (beginner) story, which was put up as a sample of the series.

ふぁんた時間 (Fantasy Time)
More of a listening resource than a reading resource, but that’s good to have too! Each story has a link to its corresponding Aozora entry, so you can read along while listening if you like, or read the story first if you prefer. (This may also be a good way of finding more reading material by the same authors on Aozora, which I think is a hard site for a beginning reader to find suitable content on.) Stories with a green button marked 立ち読みする have lovely illustrations and text to go along with the audio.
These are generally more complicated than stories from Hukumusume or the Digital Picture Book Site.

ゆめよみおはなし ひなたぼっこ (Dream-Reading Storytime: Basking In The Sun)
This used to be available as a podcast, but I can’t tell if it is anymore or not. It’s a mother and her son reading stories from different sources such as Aozora and 福娘童話集, and it’s really charming! There are links to all the stories, so you can read along.

青い鳥文庫　ためし読み！(Aoitori Bunko: Trial Reading!)
Aoitori Bunko is a line of classic books and original fiction aimed at children between around 3rd grade and 6th grade; I would estimate most of their books are level 6 or so. This page has previews of dozens of their books, so check it out if you are a fairly advanced reader and looking for a new book or series to try!

マジック・ツリーハウス 立ち読み (Magic Tree House: Trial Reading)
The Magic Tree House series has been translated into Japanese, and you can read around a dozen pages from each of the books here, so you can test them out and see if they’d be at your level and fun for you.

「いろいろな場面／「４こままんが」(Various Situations / 4-Panel Manga)
I don’t know about the rest of you, but I find 擬音語 (sound words) and 擬態語, (emotional state words) to be extremely hard to learn just from context, so one of my friends was kind enough to find this site for me. It presents those kinds of words in context and in comics.

青空文庫 (Open Air Library)
Aozora is similar to Project Gutenberg, with impressive amounts of public domain material freely available. As far as extensive reading goes, though, I think it’s really only likely to be of use to someone already reading at a fairly high level (level 5 and up by my classification system), and they use the ruby tag for furigana, which displays kanji readings in parentheses if you’re using a browser that doesn’t support the tag. (Chrome, Safari and Internet Explorer support it.) If you’re not yet at the level where you can read advanced material without decoding it, I think even the time spent trying to navigate this site to find something interesting and suitable to read would be better spent elsewhere. Still, if you can handle advanced material without decoding, it might be time to try some authentic literature here. You could start with 宮沢賢治 (Miyazawa Kenji), who wrote children’s stories such as 注文の多い料理店 (The Restaurant of Many Orders), or 新美南吉 (Niimi Nankichi), whose 手袋を買いに (Buying Mittens) is a classic.

近代デジタルライブラリー (Digital Library from the Meiji Era)
I suspect there is fun to be had here if you are not only a fairly high-level reader, but also patient with the interface and able to cope with unexpected things like old-style kanji and orthography and horizontal Japanese read right to left. There’s Japanese kids’ books in the 913.8 section (use the 部分で検索 link to get there), you could search for stories you already know like 桃太郎, or start with this page about children’s stories, from the announcement of the merging of the 児童書デジタルライブラリー (Juvenile Literature Digital Library) with this site. I can’t say I’m seriously recommending this for most extensive readers, even if there are children’s books — I just think it’s so amazing that something like this is available to the world.

NHK NewsWeb Easy
News written in easy Japanese. My impression is that this is the most simple to read of the three news sites I’ve linked here, and it also has audio for the articles.

こどもアサヒ (Children’s Asahi)
News articles written for grade-schoolers and middle-schoolers from the Asahi Shimbun’s sister paper, こどもアサヒ. If you have been reading for a while and want to start practicing to read adult newspapers, this would be a good place to start, but I wouldn’t recommend it for a beginning reader.

毎日小学生新聞 (Mainichi Grade-Schooler Newspaper)
Articles and so on for grade schoolers from the Mainichi Shinbun. Again, I wouldn’t recommend this to someone who’s just starting extensive reading, but if you’re familiar with the basics, give it a shot!