Nitroplus Co., Ltd., stylized as nitro+, is a Japanese visual novel computer software company that has developed a number of visual novels, including eroge. They have also have been collaborating with TYPE-MOON (another developer) to create the light novel series, Fate/zero. Their works usually have dark themes such as reanimation of the dead and murder. They also have a branch of the company called Nitro+Chiral, which focuses on Boys' Love visual novels.
Source: Wikipedia, "Nitroplus", available under the CC-BY-SA License.