Mission Improbable is a singleplayer mod for Half-Life 2: Episode 2. In it, Gordon Freeman is tasked with re-activating a Resistance listening post. From there, things quickly escalate.
Closest to Episode 3 as you're gonna get. You perfectly managed to capture the look and feel of half-life. It just feels like Valve made it. Which says a lot. This is the quality a game should have let alone a mod. Another person that should be hired by Valve :D