Some notable mods for Half-Life and Portal games, such as Missing Information, are based on cut content or concepts found in the game's files or in the Half-Life 2 leak.
Although Valve have a long history of hiring from mod communities, some mods such as Natural Selection, The Ship, and Black Mesa have been developed into standalone releases or franchises by independent teams without Valve's involvement. Other Half-Life mods, such as Team Fortress Classic (originally a Quake mod redeveloped on the GoldSrc engine) and Ricochet, were created and distributed by Valve themselves. Some original mods for Half-Life games, such as Counter-Strike and Day of Defeat, have since had their rights acquired by Valve and turned into standalone releases. The Half-Life and Portal series has spawned countless user-created modifications, or mods, including many that have become highly popular in their respective communities.
