Publishers of Manga
Japanese Publishers
The english page for Kodansha, Kodanclub, offers a wealth of information regarding the series availble in Japanese through Kodansha. There are even manga previews at Kodanclub that display a translation when your move the cursor over the Japanese! ^^
Shogakkan publishes many things including manga. ^^ They also publish a wide variety of magazines including some of the bigger names in the genre, the "Big Comic" series (there are several versions) and Bestucomi, a shoujo anthology.
Hakusensha J
One of my favorites, mostly just because they happen to own some of my favorite series... ^^ With 6 magazines (LaLa is one that I collect ^^) and many serialized stories all for the most part being shoujo, Hakusensha is very much a shoujo publisher. They offer nice portals to their magazine pages and have a solid search engine for their series coupled with s-book, a Japanese online bookstore.
English Publishers
ADV has, until recently specialized in releasing anime and scifi DVDs, but they have picked up a few manga. They do a lovely job, espeically with Full Metal Panic, which is printed a bit larger than average, and every issue has colour pages, even the color manga pages that really only appear in manga zasshi in Japan, are in color. I love them for that... ^^
This is a newer publishing group, they only have a few series "under their belt" so to say, but they do lovely work. I recently bought the first book of Juvenile Orion, and it included a colour page, translation notes, character webs, and info on the card game the series developed from all on heavier weight paper. Color pages are very rare so I'm always happy to see them. ^^ Even with these features, Broccoli's manga sells for $9.99. Their site also provides a store full of goods from the series they carry!! ^^
Dark Horse is generally an action genre publisher, but do carry a few shoujo titles among their many shounen and seinen type stories. They publish a lot of American comics as well as manga. Classics such as Akira and Astro Boy are translated through this company.
Tokyopop has ever since their beginning, carried a lot of shoujo titles, for quite some time they were really the only choice for selection in the shoujo style. By getting the rights for the Sailor Moon manga as the series was taking off on American television, Tokyopop really got a foothold in the business. Currently, they carry a large variety of manga, but still seem retain a slim majority of shoujo titles. They also release "Cine-Manga (c)" which are full color version of a variety of TV shows, including anime and American cartoons.
Viz currently owns two of the biggest anime/manga magazines in America, Animerica (generally anime and manga oriented, recently have added in more Japanese culture spotlights as well) and Animerica Extra (which is a manga anothology). Viz has always done nice work and for quite some time retained a larger format for their books; at that time, however, Viz manga was more expensive, running at about $15 per. Now they have lowered thier prices to a competing level just five cents under TokyoPop's prices (once the lowest on average) currently many newly released Viz comic cost about $9.95. With any publisher, however, proces may vary a bit if the manga has any special features (such as TokyoPop's release of Clover by CLAMP which features smei-transparent dust sleeves and colour pages and inside covers, those ran for around $15.00).