Lighting

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Guides

Adding Light - A beginners tutorial on how to set up a basic light entity.
Intermediate Lighting - A tutorial dealing with all the light sources in more detail.
Advanced Lighting
Constant-Linear-Quadratic Falloff - An easy-to-read explanation of the old C-L-Q light attenuation system.
HDR - High Dynamic Range Lighting.

light_dynamic - a combination point light and spotlight that can be moved, turned, and adjusted.
npc_spotlight - a spotlight NPC capable of tracking targets.
env_cubemap

The following entities are primarily just visual effects from light, meaning they do not cast light on their surroundings.

env_sprite — entity used to create omni-directional glow or flare effects.
point_spotlight — a moveable spotlight (directional) glow, with an optional dynamic light source.
env_lightglow — an asymmetrical glow (for areas of contrasting light).
The models/effects/vol_light001 and models/effects/vol_light002 textures, used for volumetric lighting.
prop_static — This entity can be used together with the models/Effects/vol_light.mdl model to simulate basic volumetric light.
env_sun — visual effect of the sun in the sky.
env_particlelight — only lights particles (such as those produced by env_smokestack).

For values to accompany the stock skybox materials, see Sky List.

Naming a light makes it more expensive. Only name lights when you need to. Multiple lights sharing the same name are cheaper than lights with separate names.
light_dynamic is especially expensive, and will not even show up on some systems
The basic light entities do not come with a visible representation (e.g. a lightbulb). For that you need a prop. See HL2 light props for a list.
The basic light entities do not come with a "glow" as you'd expect to see in a foggy or misty area. To provide this, either compile and run the map with HDR enabled, or add a point_spotlight (with dynamic lighting off unless needed) or env_lightglow entity.
Maps will not be lit unless you run vrad (or equivalent). Vrad will not run properly and will not calculate realistic light bounces unless the level is free of leaks.
Several textures provide their own light & can do so using lights.rad in a mod's directory or the one in sourcesdk/bin.
Moving brushed-based objects will not change the way they are lit. Their lighting will be calculated only according to their positions in Hammer. (e.g. a brush in a dark room will not become bright if it is brought into a bright room.)