Tremulous

Tremulous is a free and open source team-based first-person shooter with real-time strategy elements. The game features two opposing teams: humans and aliens. Each team must attack the enemy’s base and team members, while defending their own base. The game has been downloaded over 2,000,000 times and was voted Player’s Choice Standalone Game of the Year in Mod Database’s Mod of the Year 2006 competition.
During a match, most players engage in fighting the enemy, while others maintain the base and construct new structures. In order to destroy the enemy, humans utilize various weapons, armor and other upgrades, while aliens may evolve into more powerful classes, each of which possesses unique abilities.
The teams do not have access to all possible upgrades and buildings at the beginning of the game. Each team must reach a threshold of frags in order to advance to the next developmental stage. The point at which a stage change occurs varies dynamically depending on the number of players on each team. Each new stage brings more upgrades for the humans and more classes for the aliens.
In addition to requiring a certain stage, each human item or alien class must be purchased using currency earned in game. The aliens are awarded frags for killing their foes which may be used to evolve (these points are commonly called evos by players). The Humans gain credits. Players earn currency by killing enemy players or by destroying key enemy structures; the amount of received currency depends on what class the opponent was (or what structure was destroyed) and how much of the target’s total damage was dealt by the killing player. As a mechanism to encourage builders, who naturally don’t engage in combat, players are also rewarded currency every two minutes by simply staying alive.
Builders are responsible for construction of new buildings and base maintenance. Each building takes up a certain number of build points available to each team. The number of build points can vary from map to map and server to server, limiting how large bases can become. To reduce the likelihood of the match ending in a tie, most games enter a sudden death mode ten to fifteen minutes before the draw/tie time limit. This is a period during which the building of most or all structures is prohibited, allowing attackers to more easily overwhelm the defenders.

Download link: http://tremulous.net/files/
(we will play on the 1.1.0 version)