Japanese homebrew shooters, or doujin shmups, form a subculture of developers and dedicated players in the indie gaming scene. It's a massively confusing (but not-so-multiplayer) world of strange games, websites with squiggly characters, and obscure names that have become the subject of fanboy worship. If you've played Warning Forever, Clean Asia, or even Cave Story creator Pixel's new project Guxt (a work-in-progress demo) you're off to a good start.
The Independent Gaming Source has published a quick guide to the world of doujin shmups. Everything is divided into digestible bits of information that actually makes sense to the doujin n00b. And once you know who Kenta Cho is and why you should put him in your will, an entire universe of free indie shooters is at your disposal.