Half-Life 2 Rebalanced is a mod that revamps the gameplay and balance of Half-Life 2. It aims to give HL2's gameplay more variety, make it more difficult and engaging, and to polish and improve the game wherever possible. With the ultimate goal of recreating the intense yet fun gameplay of the original Half-Life.

Featuring:

Revamped weapon balance to encourage more varied weapon usage.
Smarter and more engaging enemies that can hold their own against the player.
New hi-res crosshairs for each weapon.
Lots of bugfixes and general polish to existing features.
Additional dynamic lights for common effects/objects.
Grenade cooking and other QOL additions to help improve the player's experience.
Expanded capabilities of enemies and other features to add more gameplay variety.
New toggleable challenges that can alter the gameplay to be even harder.
More Convars and a new difficulty-specific .cfg system to substantially increase customizability.

Modding HL2R:

Half-Life 2 Rebalanced is also fully moddable due to being completely open source. That means mods can base themselves off of, or use code from HL2R without needing express permission. Making it entirely possible for modders to create things like compatibility patches or overhauls for HL2R.

The Sourcecode can be accessed here for anyone to use or edit it for any purpose.

Installation:

Once the zip is done downloading, open it and extract all the files into your sourcemods directory in Steam/steamapps/sourcemods/

From there restart Steam and the mod should appear in your library. Just make sure you have both Half-Life 2 and its Episodes installed.