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Homeworld 2 is the 2nd game in the Series by Relic & Sierra. The Homeworld series is a Scf-Fi space-based RTS. It was (and largely still is) unique in it's integration of easy-to-navigate 3D battles.

MDK2 is a shooter from Bioware and Interplay, and the sequel to the classic shooter MDK from 1997. Kurt Hectic, a futuristic commando with a stealth suit and sniper helmet, returns to do battle with malevolent aliens once again. New to the series are playable characters Max, a six-legged dog who can wield four weapons and fly with a jetpack, and Dr. Hawkins, a mad scientist who must gather pieces of junk and combine them MacGuyver-style to create weapons.

Psychonauts is an action game from Double Fine Productions and Majesco, with design by Tim Schafer, famed designer of Full Throttle and Grim Fandango. You play Raz, a kid attending a summer camp, where students are trained to harness their pyschic powers for combat and intelligence purposes. You must develop your own mental abilities through mental training, and use your abilities to solve a kidnapping case in the summer camp.

Command & Conquer: Tiberian Sun, is a sci-fi themed RTS from Westwood and EA, and a sequel to the pioneering Command & Conquer. You play as the commander of GDI or NOD, and continue to wage apocalyptic war on the other side.

3rd person adventure game with a Sci-Fi protagonist. Very story driven with an emphasis on cinematics.

A top-down third-person shooter. You are stationed on one of Jupiter's moons which was recently colonized, and are called in to investigate the cause of a power outage. Aliens threaten to overwhelm a nearby colony. Only you stand in their way.

Rise of Nations: Rise of Legends is a real-time strategy game for the PC made by Big Huge Games, and published by Microsoft. It is a spin off of the popular game Rise of Nations, released in May 2003. However, rather than being a historical game, it is based in a fantasy world, where technology and magic coexist. It was released on May 9, 2006 in North America and on various dates around May 26, 2006 in the rest of the world.

Half-Life 2 is an FPS from Valve Software, and was one of the most highly anticipated sequels of all time. You return as Gordon Freeman, the strong and silent-type physicist, and you must do battle with an alien race of enslavers to liberate the surviving remainder of humanity on earth.
Also applies to Half-Life 2 Lost Coast free downloadable level.