The Mobility Mod adds wall-running straight out of Titanfall 2 to Half-Life 2. With it, you can even kill enemies by jumping on their heads.
While a version of the Mobility Mod has been around for a while now, the modder behind it has now released a v2 that is also compatible with both of Half-Life 2's episodic follow-ups. Among the changes are new melee controls, so you can perform rocket-boosted surge attacks and multi-target crowbar slashes. Using the surge attack in the air will even let you fly, at least for a moment.
You can download v2 of the Mobility Mod from ModDB.