DotA 2 is based upon the popular WarCraft III mode DotA (Defense of the Ancients), and it is a strategy multiplayer online battle arena game where you control a hero which can advance through various levels and gain new skills and abilities, buy upgrades, items and have to defeat the enemy heroes and structures. (Image credit)
This is a popular, free cartoonish shooter in which you can pick from a range of different classes and weapons and go around destroying your enemies.
No More Room in Hell is a horror-themed shooter where you play in an online cooperative mode to defeat hordes of enemy zombies.
Cannons Lasers Rockets takes place in 3D space environments, and you control a ship which can move in a horizontal plane, you have three weapons at your disposal and have to defeat the enemy ships and base.
Champions of Regnum is a MMORPG (Massive Multiplayer Online Playing Game) in the same style of World of Warcraft. You can choose from various characters and go around completing quests and fighting enemies in three different, larg-scaled realms. (Image credit)
Dwarfs is a 2D strategy, arcade-like game with various modes and expansion packs. In Dwarfs you have to built up your kingdom, mine for gold, protect your kingdom and survive in the meantime. (Image credit)
The World of Goo Demo is free to try on Steam. World of Goo is really addictive and fun, and it's all about building structures that will be impacted by factors like gravity, in order to reach certain places, using Goo balls. The demo includes the first four chapters.
Star Conflict is a 3D, graphics intensive massive multiplayer space simulation with beautiful scenery, battles and exploration elements. (Image credit)
Reversion is a Sci-Fi point-and-click adventure game set in 2035, where the main character has to gather clues on what happened to his memory.
WAKFU is a massive multiplayer online role playing game with an anime theme, which features tactical battles in a fantasy universe. (Image credit)
Rising Angels: Reborn is another anime-themed game which takes place in deep space, featuring single-player missions. (Image credit)
Robocraft is a massive multiplayer robot vehicle shooter. It's still in development, but you can play it in early access. According to the description, "if Minecraft smashed straight into World of Tanks you'd get Robocraft". Looking very promising, Robocraft allows you to build your robot, equip it with jets, guns, wheels, and proceed on destroying your enemies.