Half-Life: Peaces like Us is a singleplayer modification for Half-Life. Story: Mankind found an entrance to XEN world and an investigation team met with the uncanny aliens in there world. Then, mankind succeeded in becoming allies with them. Later, a scientist team built a joint laboratory in the world of XEN. But, something bad was approaching... Info: An alien is a friend. Don't shoot it! Hint: There is a secret in a certain map. This place seems to fall.
One of the earlier Single Player Half-Life mods, this one is quite memorable. Several of the maps were quite innovative at the time and the story turned out to be a little bit of foreshadowing.