Natural Selection is a mod for Half-Life released on Halloween of 2002 that incorporates elements of real-time strategy into the normal first-person shooter action.
Essentially there are two teams: humans (space marines) and aliens (Kharaa). The human team gets a commander that plays the game from a top-down perspective much like an RTS giving orders that are then carried out by players as well as building structures and keeping track of things. All the other players function a lot like they do in most shooters except they build the buildings the commander places as well as fighting aliens. Aliens, on the other hand, do not have any commander. They get something called hive sight which lets them see each other as well as various buildings and enemies at all times through walls and such (basically sprites marking relative position and color-coded with some text as well).
The overall goal is to destroy the other team and their ability to keep coming back. For humans this means all the aliens and all three of their hives, for aliens all of the humans and the command console inhabited by the commander. In the intervening time there are resources to be gained, area to control, new forms for aliens to evolve into, new weapons and armor for humans to research and purchase, evolutionary traits for aliens to select and so on. In the later stages of the game it's very likely to be fighting cloaking, regenerating, silent aliens shooting acid bombs at you or marines with jetpacks or power armor carrying heavy machine guns and laying waste to all you hold dear. Teamplay is thus an essential component and without it you're toast.
Even an exhaustive writeup would fail to capture all that there is to see and do in this mod. The main site is located at http://www.natural-selection.org