Source

Source is a 3D video game engine developed by Valve Corporation. It debuted as the successor to GoldSrc with Counter-Strike: Source in June 2004, followed shortly by Half-Life 2 in November. Source implemented many new features and optimizations that GoldSrc did not have.

Rendering

 * Bumpmapping.
 * Phong shading.
 * Moving lights.
 * Water with refractions, realtime reflections, and flow maps.
 * Advanced particle system that can emit sprites or models.
 * Deformable terrain.
 * 3D skyboxes extend the horizon and add parallax on distant objects.
 * Static reflections.
 * Cascaded shadow mapping.
 * Cheap render-to-texture shadows make objects look like they occupy a real space.
 * Effects such as smoke, fog and rain

Characters

 * Characters that run, fly, jump, crouch, climb stairs and ladders, and burrow underground
 * Eyes can move separately from heads.
 * Support for many spoken languages.
 * Battle AI allows squads of AI characters to operate together, know when to advance, retreat, lay cover fire, etc.

Physics

 * Objects are free to move around in the world and rotate freely.
 * Ragdoll physics.
 * Vehicles, with individually simulated wheels. Vehicles have individually tunable parameters such as horsepower, gearing, max speed, shift speed, tire material, tire friction, spring tension/dampening etc.

Sound

 * Digital Signal Processing.
 * 16-bit wav standard.
 * Textures can dictate how much sound they absorb.

Programming

 * Mainly written in C++. Many games have their source code available publicly.
 * Input/Output system allows level designers to control objects much more powerfully than before.
 * Squirrel and Lua scripts offer even more complexity.