Forget Gordon Freeman. If you really want to make the Combine suffer, you'll need to enlist the talents of the tough-as-nails space marine from DOOM.
In the pantheon of first-person shooters, there are two shining examples that always rise above the competition in terms of their importance to the genre and the gaming industry as a whole: id Software's DOOM and Valve Software's Half-Life. The former may not have invented the first-person shooter genre, but it certainly deserves credit for popularizing the concept. Half-Life, in turn, demonstrated that FPS games need not be mindless, run n' gun affairs, and that it was entirely possible for a talented development team to tell a gripping, engaging story, while also granting players their fix of shoot 'em up action.
Despite their similarities, DOOM and Half-Life are divided by years of time, and disparate developers. If it weren't for the efforts of devoted, clever modders, the two titles would never meet. Fortunately the PC platform offers players the freedom to do basically whatever they'd like and as a result a resourceful fan of both properties has melded DOOM and Half-Life 2 into a single, nostalgia-heavy modification.
Though not yet completed, you can see the current status of the title in the clip embedded at the top of this page. Owing to Valve Software's generous support of the modding community, putting this project together wasn't as difficult as you'd imagine. Instead of necessitating hardcore, deep-level alterations to these games' code, the people behind this mod built the whole thing within Garry's Mod, a fan-favorite sandbox "game" that basically gives players a toolbox compatible with any element ever created within Valve's Source Engine, and urges them to build whatever crazy creative efforts they can possibly imagine. Normally this results in hilarious, eerie videos that place normally stoic characters like Half-Life's G-Man in ridiculous situations, but in the case of this clip we're shown just how extensive the powers of Garry's Mod truly are.
The result is a mashup between Half-Life 2 and DOOM. We're shown the familiar surroundings seen in the former title overlaid with the protagonist, weaponry and overall sensation of chaotic violence found in the latter. Gordon Freeman was an interesting character, but he doesn't hold a candle to the brute force of a space marine toting a double-barreled shotgun and laying waste to any Combine soldiers or Ant Lions stupid enough to get in his way.
The best news here is that the mod is almost completed and merely needs final bug testing before it can debut on the Steam Workshop. PC Gamer claims that the modification should be available at some point this week, though as of the time of this writing it remains absent.
Source: PC Gamer