Doujin music is essentially music that is doujin. Doujin works are essentially self-published works and forms a fairly large part of the Japanese otaku community and experience. Doujin works encompass everything from games to music to anime to manga. Among the most famous doujin works is Touhou Project, the shrine maiden shooting game created by ZUN.
So what kinds of stuff can you find under this vague banner?
Touhou. At this point in time, Touhou is a doujin phenom. The original works themselves are doujin games and have spawned an immense amount of music, manga, animations, and other weird derivative works. In addition to dominating events like Comiket, Touhou has its own set of doujin events, Reiteisai.
ex.:
EastNewSound and nayuta - 緋色月下、狂咲ノ絶 -1st Anniversary Remix-
東京アクティブNEETs - Royal Greenwich
岸田教団&THE明星ロケッツ - ただ凛として and 芥川龍之介の河童
Vocaloid. While there are a few Vocaloid producers who have been lucky enough to eventually sign on with a record label, most Vocaloid producers that produce albums will fall under the doujin label, at least for a while. Like the Touhou people, they also have their own event, The Voc@loid M@ster, although they don't quite take over Comiket the same way.
ex.:
buzzG - かくれんぼ
40mP - 夢地図
DECO*27 - トリノアイウタ
Music for other doujin works. Creators of doujin works do not have much money, so they go and cut deals with other members of the doujin community. As a result, the music for a lot of these works come from doujin music people. It might seem strange that people would want to buy doujin music from doujin works, but let me remind you that Higurashi no Naku Koro ni and Umineko no Naku Koro ni are both doujin games by 07th Expansion and features music from a large number of doujin artists, the most well known of which are dai and zts.
ex.:
zts - dreamenddischarger
dai - You
Raito - Boundaries of the Sky - Commanding View of the Scenery
Remixes. Remixes of music from games and anime is a cornerstone of doujin music and of doujin works themselves. After all, doujinshi are essentially fanfiction, or at a more meta level, involves the use of elements of the original works outside of their original context. There are plenty of groups that focus on doing arrangements of music from certain games and such.
ex.:
marasy - Fire◎Flower
CROW'SCLAW - エミヤ
グラサンねこ - worldenddominator
Original stuff. A lot of the doujin music circles are quite adept at composing their own works in addition to building off of existing works. There are plenty of doujin vocalists who work with doujin composers or produce stuff on their own.
ex.:
バルシェ & minato - Myself
PolyphonicBranch & やなぎなぎ - 砂の城
that clear みたに yonji バルシェ perfumen(白服・眼鏡・仮面2) プリコ ひらどん ill.bell くりむぞん ほんこーん halyosy トゥライ 蛇足 実谷なな にっとメガネ che:櫻井 ひと里 てん リツカ らっぷびと 赤飯 ピコ [TEST] is 事務員G 地味侍 - Smiling
And this is where I tell you that it'll be almost impossible to buy any of this stuff. Because of their self-published nature, doujin materials are extremely rare unless they're by one of the bigger circles. If it's from a bigger group, if you live in Japan, you might have a shot at ordering something from Tora no Ana or something. If it's from a smaller circle, then your only chance at getting something is at an event. After that? No more.
So let's talk about music that you've never heard of.