Darkwood: A recently released, top-down perspective horror themed title from Acid Wizard Studio, provided with no cost with both the reasoning for players who cannot afford its price, and to discourage key distributors. (Note: The webpage linked is directed towards Acid Wizard Studio's story of development, posted upon Imgur. The last image provides a link to download. You will require a BitTorrent client to aquire the files)
Hero: A retro inspired action and adventure platformer, including a basic MIDI developed soundtrack, monotone visuals, and six levels for the player to navigate through.
Hero Core: A sequel to "Hero", introducing a larger world for the player to explore, lenient progression, and difficulty settings.
Minetest: An open-source sandbox game, based from similar titles including "Minecraft" and "Infiminer". It has a focus towards its modification support and community development. Also available as a downloadable .apk for Android users (Note: On the linked webpage, it mentions that it is available upon the "Google Play Store", but to take caution due to possible false uploads).
OFF: A graphically simple role playing adventure game, with a notably gloom theme throughout. Due to the possibilty of unintentionally mentioning too much about the story, I simple suggest to play about with it.
OpenRA: An open-source, real-time strategy, command and conquer engine, featuring multiple games based from classic title series including "Red Alert", "Tiberian Dawn", and "Dune 2000", alongside modification support, multiplayer, and more.
OpenTTD: An open-source simulation game based from "Transport Tycoon Deluxe". Features isometric 16-bit graphics, multiplayer, support for modifications, and more.
osu!: A rhythm game based from "Osu! Tatakae! Ouendan". Features a focus upon music, rhythm, modification support, and more.
SCP - Containment Breach: A horror game based upon various elements from the "SCP Foundation" within randomly generated corridors.
The Dark Mod: A Gothic and Steampunk themed stealth game, in which was once a mod worked based from "Doom 3" that has now become standalone to play.