This will be a roundup of all the best games for Windows. All of the games are free to download, and released under an open source license.

First-Person Shooters - video game genre centered around gun and other weapon-based combat in a first-person perspective.
Platform Games - video game genre and subgenre of action game.
Sports Games - video game genre that simulates the practice of sports.
Racing Games - take part in a racing competition with any type of land, water, air or space vehicles.
Simulation Games - describes a wide-ranging super-category of video games, generally designed to closely simulate real world activities.
Board Games - source of social interaction, as they challenge the mind and, most importantly, they are great fun to play
Educational Games - designed to teach people, typically children, about a certain subject or help them learn a skill as they play.
Chess Games - a wide range of chess-playing software for Windows, which can defeat most master players under tournament conditions. Play against a computer engine or online with fellow humans!
Roguelike Games - Roguelike is a sub-genre of role-playing games. It literally means “a game like Rogue”. Rogue is a dungeon crawling video game, first released in 1980.
ASCII Games - extremely basic graphics, but with many redeeming qualities beyond evoking fond memories of the early days of computer gaming.
Card Games - plethora of card games available, with families of related games.
Music Games - video game where the gameplay is meaningfully and often almost entirely oriented around the player’s interactions with a musical score or individual songs.
Game Engines - software development environments designed for people to build video games.