Mods like MMod or the Cinematic Mod enhance the visuals, lighting, and gunplay to make the game feel modern.

Garry's Mod (GMod) started as a simple mod and became a global phenomenon for building and physics experimentation. Where to Download (Safe & Free)

Most major mods require you to have "Source SDK Base 2013" installed (found in your Steam Library under Tools) to run properly.

Manual installations usually go into your Steam/steamapps/sourcemods directory. To help you find the right version or files, let me know:

The easiest way to install mods for many Source-engine games.