A Guide to the Best Open-Source Games for Ad-Free Gaming

In a world of games filled with ads, trackers, and microtransactions, the world of open-source gaming offers a refreshing alternative. These games are not only free to play but also provide access to their source code, making them a great resource for both gamers and aspiring developers. Below is a curated collection of recommended open-source titles across various genres.

Strategy and Simulation

This is one of the most mature genres in open-source gaming, with many long-standing and polished titles.

Battle for Wesnoth: A highly-regarded turn-based fantasy strategy game with numerous campaigns and factions. A particularly great tip is to try the user-made add-ons, especially the RPG-style maps where you control and level up a single powerful unit against hordes of enemies.
0 A.D.: A real-time strategy game of historical warfare and economy building, often compared to the Age of Empires series.
OpenTTD & Simutrans: For fans of management sims, OpenTTD is a beloved and highly-addictive open-source clone of Transport Tycoon Deluxe. Simutrans offers a similar but distinct take on transportation simulation.
Freeciv & Unciv: These two projects are faithful recreations of the Civilization series. Unciv is particularly noted for its lightweight design and excellent mobile support.
OpenRA: An engine recreation that allows you to play classic Command & Conquer titles (Tiberian Dawn, Red Alert, Dune 2000) on modern systems with an improved interface and online play.
Warzone 2100: A 3D real-time strategy game with a focus on technology, radar, and vehicle design.

Roguelikes and RPGs

If you enjoy deep, challenging, and replayable experiences, the open-source roguelike scene is thriving.

Dungeon Crawl Stone Soup: A classic roguelike praised for its design philosophy, which aims to eliminate tedious tasks. It includes helpful features like auto-explore and a search function for items you've left behind.
Cataclysm: Dark Days Ahead: A complex and deep post-apocalyptic survival roguelike with immense crafting and customization systems.
Shattered Pixel Dungeon: A popular and accessible roguelike that is especially well-suited for mobile devices.
The Ur-Quan Masters: The official open-source port of the classic sci-fi game Star Control II, blending space exploration, combat, and a compelling story.

Action and Adventure

Xonotic: A fast-paced, arena-style first-person shooter in the vein of Quake and Unreal Tournament. While the player base can be small, it delivers classic FPS action.
The Dark Mod: Originally a mod for Doom 3, this is now a standalone stealth game heavily inspired by the classic Thief series. It boasts a large library of community-created missions.
Pioneer Space Sim: An open-ended space exploration game set in a procedurally generated galaxy, offering a mix of combat, trading, and exploration reminiscent of Kerbal Space Program and No Man's Sky.

Further Exploration

For those who want to see a simple game built from the ground up, Dungeon Maze is an example built with vanilla HTML, CSS, and JavaScript, without any game engine. For a more exhaustive list of titles, the Wikipedia List of open-source video games is an invaluable and comprehensive resource.