Lighting

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Entities

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

point_spotlight — a moveable spotlight glow, with optional dynamic light.
The models/effects/vol_light001 and models/effects/vol_light001 textures, used for volumetric lighting.
Todo: Is volumetric lighting just a texture effect, or does it emit a faint bit of light?
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; does not actually provide light.
env_particlelight — only lights particles (such as those produced by env_smokestack).

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 (eg, 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 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.
Experiment by burning light onto motion enabled entities/objects. i.e. func_rotating with shadows disabled may look odd, func_tracktrain spawned with normal light, etc.