its a dojin comic about witches. they also have some PC games and other things. its mostly all comitek stuff, very popular. tsukihime started out like this also.
Actually, the other way around... started as a fan-made game (first one a brick-buster thing, rest are shmups), went on to become really popular and now it's a lot of everything. This is one of many doujins. Check out http://www.shrinemaiden.com for more info.
