Day of Defeat
Day of Defeat (DoD) is a Mod (game modification) for the Half Life game engine. People have been writing their own maps for game engines to run on since Doom. A Mod actually changes the way the game plays. A better way of describing a Mod is that it is a new game written using the existing infrastructure of a game.
Counterstrike is a famous Mod of Half Life (HL), it uses the game engine to produce a game that plays very differently to the original. A less well known but well loved Mod is DoD, which uses the game engine with new graphics to have a team based game based in World War II, Allies verses Axis.
I recall downloading an early version and thinking it was rubbish. This changed with the latest version running on HL1 engine, which was very bright and fun. The current Source (HL2) version is different but the same and enjoys a large but slowly declining following. The British Allies vanished with the Source version, so is only American vs German troops on a huge number of third party maps. It is mainly a capture the flag game, but with variations on the theme with the need to destroy objectives (one way capture) and even the “Orange maps” which do away with the fancy scenery for a minimalist approach. Even then, you can have an Orange capture the flag, or a “Gun Game” where individual players on both sides begin with the same weapon, but advance through the weapons with each “kill” and the round ends when a player has used all the weapons.
Online clans and communities have prolonged the lifetime of DoD, although there seems to be much less interest in competetive leagues of DoD than there once was. Worth a look.
