Gunman Chronicles is a futuristic first-person shooter video game originally created as a mod by the now defunct Rewolf Software. Gunman Chronicles was originally a Quake deathmatch mod named Gunmanship 101, then it was moved to Quake II's engine before becoming a Half-Life mod. At the Half-Life Mod Expo in 1999, Gunman Chronicles stole the show and Sierra approached Rewolf to make a retail version. After significant work, Rewolf was given some office space, funding and a mapper (Jeff Lane) by Valve Software to help complete the project. It was released as a standalone game.
Source: Wikipedia, "Gunman Chronicles," available under the CC-BY-SA License.