Boys' Love (BL) is a commercial category of women's manga focusing on male/male relationships. The term used in Japan is "BL" or "Boys' Love". Western fans (and even publishers) tend to use yaoi - incorrectly - as a catch-all for any Japanese story that focuses on male/male relationships, but yaoi is a specific, slashy type of BL (there is much confusion over terms among Western fans: visit Aestheticism to learn why certain terms are inappropriate).
I don't read BL myself so I can't offer much in the way of recommendations. Instead I suggest Yaoi Suki and the Yaoi Suki Wiki as excellent resources for reviews, release dates, and company information.
Books are complete in 1 volume unless indicated otherwise. No ratings here as almost all BL are Older Teen and up. The rating is available on both the publisher and RightStuf websites.
Focus: Well-known, longer series. Mature titles.
CPM has gotten a rep for censoring many of its BL manga while charging $16.99 USD per book for below average printing quality. Due to financial problems in 2005-2006 they have also been late on their titles. Known censorship: Golden Cain: missing 7-page chapter of incestuous boy love fantasies. Kizuna: omitted underage boy love kiss. Selfish Love: missing 12-page chapter of boy love sex - CPM has publically stated on their website that some printings of volume 1 are missing 3 pages due to a printing error, and that you may receive a free replacement copy.
Links are to the book page on Be Beautiful and the RightStuf item page, respectively.
Focus: Mature titles, though not necessarily hardcore
Links are to the book page on Blu Manga and the RightStuf item page, respectively.
So far Broccoli has only published one proper BL title, Until the Full Moon (Full Moon ni Sasayaite). They've also published the pretty boy shoujo title Aquarian Age - Juvenile Orion.
Focus: Mostly mature, generally hardcore titles
DramaQueen is a small specialty publisher known for their high production values and dedication to the BL market, in addition to some very interesting girls' manhwa and manga. From their About Us page it sounds like DramaQueen is an amateur group turned professional publisher, which is just too cool because, not only are they the first female fan-run publisher, their books are some of the best in the business in terms of production value. DramaQueen is primarily a BL publisher, but they've quickly expanded into shoujo romance from both Japan and Korea. Due to the graphic nature of their BL books, they are not (currently) distributed in bookstores and are available online thru their website, RightStuf, Amazon, and Barnes & Noble.
Links are to the book page on DramaQueen and the RightStuf item page, respectively.
Focus: Mostly lighter, teen-friendly titles
Digital Manga Publishing publishes a number of BL manga under the Juné Manga imprint (formerly Yaoi Manga) and early on established themselves as a premier publisher of BL for their attention to detail, high production values, and faithful adaptations. Links are to the book page on Juné Manga and the RightStuf item page, respectively.
Media Blasters publishes teen-rated BL anime under both their main line and their Anime Works label
Media Blasters also publishes hardcore yaoi manga and anime under their hentai anime label Kitty Media (NSFW), which puts out some rather nasty hentai. Viewer descretion is definitely advised.
Netcomics is the American branch of the Korean manga publisher eComix, which publishes comic book contents as online books, mobile phone contents, along with printed books. Netcomics similarly provides their manhwa/manga in both an online pay-per-view service and regular print versions. BL is just one of the wide variety of genres which Netcomics licenses. Series debut online before being compiled into print volumes, and can be viewed for fraction of the cost of a print book: the first chapter of a book is free and the rest of the book can be read online (but not downloaded) for a charge of 25¢ per chapter. The total cost of reading a book online is usually about $1 USD. The most popular online titles are released as books. So you can still get the series you like in a proper book, and avoid wasting money on disappointing and/or badly translated series: translation seems to be a big problem with Netcomics books at the moment. The problem with Korean publishers translating their own material is that one should never translate from one's *own* language into another without having a native speaker of that language rewrite it so it sounds natural. That's what any professional American publisher does.
Tokyopop publishes a few BL and teen-rated "almost BL" books (e.g. Eerie Queerie!) in addition to their BLU line.