To all who play touhou, a danmaku (bullet curtain) game in which most characters are girls.... "It's all about power and strategy" - Dan Salterio

Okay, since most of you didn't know what touhou is, The Touhou Project or Project Shrine Maiden, is a series of manic shooters made by Team Shanghai Alice. They are notable for being successful dōjin soft because of their intricate bullet patterns, their greater focus on characters than most shooter games, and being a one-man project (ZUN).

The game also features astounding and stunning soundtracks, all composed by ZUN himself, who also does all the coding for the game - Arashi

Many people has at least interacted with some kind of Touhou, but never noticed them. Some examples are the famous video Marisa Stole The Precious Thing and Lunatic Udonge-in Overdrive

List of Official Touhou games so far :

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Touhou 1 - Highly Responsive to Prayers
Touhou 2 - Story of Eastern Wonderland
Touhou 3 - Phantasmagoria of Dim Dream
Touhou 4 - Lotus Land Story
Touhou 5 - Mystic Square

-------------------Windows age--------------------
Touhou 6 - Embodiment of Scarlet Devil
Touhou 7 - Perfect Cherry Blossom
Touhou 7.5 - Immaterial and Missing Power (Fighting)
Touhou 8 - Imperishable Night
Touhou 9 - Phantasmagoria of Flower View
Touhou 9.5 - Shoot the Bullet (Take Pictures)
Touhou 10 - Mountain of Faith
Touhou 10.5 - Scarlet Weather Rhapsody (Fighting) (Not released yet)
Touhou 11 -Subterranean Animism

There are also other unofficial Touhou games, such as Touhou Soccer and PatchCon which are very popular too.

For more infos, see Touhou Wikipedia