Studio Radi-8, the team behind the Neotokyo Mod, have released a "mini-mod" which lets you play through Half-Life 2 singleplayer with a High-Definition Lighting (HDR) effect added. HDR Lighting is a way of rendering levels of brightness far higher than usual, so it generates an effect similar to staring at a very bright light in a dark room. Put simply, it makes the lighting look better and more realistic.
Here is a before and after example of the effect this mod has on the lighting, as well as an additional screenshot:
The effect is rather subtle, but it becomes more obvious when in motion. To download the mod, visit the mod's official website. Visit this thread for more screenshots. In order to play this mod, your video card must be able to support Pixel/Vertex Shaders 2.0.