While there are innumerable websites out there with manga information, not all of them can be reliable. In cases such as Wikipedia, people are allowed to change information without permission or post information without citing the source. So while all the information can be accurate on one page, it is not true for another.
The important thing to keep in mind when searching for information is what you intend to use it for. For instance, if you are writing a paper for school, it is probably not wise to choose a random website or journal and use the data collected from there. However, some fan sites do provide their sources of information, and thus makes them more credible for academia.
On the other hand, if you are looking for reviews of manga or want to join a community so you can discuss your favorite title, then maybe the credibility of the website is not as important to you.
Below is a list of websites, grouped by site focus, for easier information navigation.
Manga News and Information Banks
Anime News Network is a great source for information ranging from the latest anime/manga news to a lexicon of common anime/manga related terms, complete with the kanji and Japanese pronunciation. The encyclopedia is a massive collection of information that includes alternate titles of the series (written in that particular language), publishing information, author's name in both romanization and Japanese, title release dates, and much more.
Comipress is probably lesser known than Anime News Network, yet the information found there is just as valuable. The article section is worth noting. The articles span a wide range of topics, all of which any fan looking for a more academic approach to manga will find interesting. If you are looking for publishing information, I would highly recommend Comipress. It provides translated weekly updates about the top selling manga in Japan.
Comipedia is a website with listings of manga magazines and publishing companies. While the information on the site is not too in depth, it does provide important information such as, initial publishing date, publishing company, and titles published in a particular magazine. For publishing companies, it lists the official sites, a brief description, imprints, magazines, and anthologies.
Anime/Manga Section at Mania.com
While I cannot really say that this is a great news or information bank, I added this to the list because of the manga comparison guide. Here, we are given a title's North American and Japanese publisher as well as it's status (completed, on-going, etc) and the number of volumes that can be found in the North American vs. Japanese editions. While this information is by no means hard to find through most of the sources listed above, it is all placed conveniently on a chart for easy access.
About.com's manga site gives a general overview of manga. From lists of series and publishers to interviews with mangaka, this site should be informative for those of you looking for basic information about a particular series/mangaka/etc.
Manga Blogs
Bento Physics is one of those sites that are a diamond in the rough and have stopped updating. Here, we have an academic approach to manga with various articles including Feminism in Shoujo Manga and Ouran High School Host Club: Let's Have Romantic Love. There are also several great links to other websites.
The purpose of this blog, as stated by the author is, "...to, hopefully, provide a comfortable and useful setting for my virtual seminar about josei manga for North American audiences, particularly focusing on public library collection development." The blog takes a more academic approach to manga along with providing the reader with a type of review. While this site is a very interesting read, there were some errors that I came across. This is an example of the fact that you should be careful about what you read on these types of websites.
Johanna Draper Carlson has a blog that, while does provide some news, is more focused on manga reviews, compiled from various authors. If you are looking for a more general review without the academic tone, I would recommend this blog.
Manga Blog is a more general site created by Brigid Alverson. It provides reviews, news, and a few interviews that are interesting. While I would say it is much more fan based than some of the other websites on this page, it is an excellent source for links other blogs and articles.