I was poking around my bloated e:\games directory the other day, and came across an old favorite: Heart of Evil. Actually, no. I was reminded of it when I read about Shellshock 2: Blood Trails on Rock, Paper, Shotgun. Vietnam? Zombies? It seems random, but HoE's got enough chutzpah (and South Park-like racial sensitivity) to make it work like nuoc mam and rice.
For starters, Heart of Evil is possibly the only game to get the escort mission right, which is good because the ENTIRE GAME is an escort mission. You must drag Barney ("I'll be god-damned if I know what his last name is!") through all six episodes, both because your character lacks the basic mental processes needed to operate the motor vehicles that take you between hubs, and because you'll feel uncontrollable guilt and commit hara-kiri if he dies. This works where every single goddamned FPS/TPS has failed miserably before it. Since each episode is broadly hub-based, you can park Barney somewhere convienent, kill everything else at your leisure, retrieve Barn, and proceed. And Barney actually can handle himself in a fight. Sort of.
What really makes this series great is that it ladles the action on old-school, dropping HL's entire weapon line-up for a more Vietnam-flavored collection. While strictly speaking the balance is iffy, every weapon is fun enough that you'll want to use it, and not just because everything else is out of ammo (being a warzone, you're usually up to your solar plexus in all the common calibers, up to and including 40x46mm LV for your M79), but because it fills a niche. And speaking of niches...
The enemies neatly fall into three categories: human, zombie, and nuisance. The humans act much as they did in Half-Life, which is of course a good thing; for some reason, there's both good Americans (who fight alongside you with the standard "walk with me/park your ass" HL NPC control) and bad Americans (it's complicated) along with the 'Cong. Zombies soak *amazing* amounts of damage (try 3 full AK mags...to the head), so blast weapons aren't so great, but dumping hundreds of bullets works; humans go down easier, but actually hurt you at range, so you can't stand there with the trigger down and dish it out. Ferreting out shooters with explosives works satisfyingly well, especially when a black-pyjama'd corpse arcs lazily through the air from a hidey-hole. Satisfaction! The enemies are surprisingly balanced; both are equally threatening, but in totally different ways - you try to maintain distance from zeds while hosing them continuously, and you try to keep hard things between you and humans while putting accurate fire and/or explosives in their direction. Things get really fun when more than one is in a battle at one time - up to four-way battles. Do you hold back and let one side get wasted, or do you try to keep one side alive (and thus dealing damage) long enough to weaken the other, so both are easy pickings? Being sneaky is beneficial; since everyone hates you the most, having two sides on you simultaneously is a world of pain.
Being a blend of Half-Life, Apocalypse Now, and early Resident Evil, episodes C-F are chock-full of bizarre key puzzles and RPG-like flourishes. If you put your crosshair on a usable object, you get a subtitle ('door,' 'roast moose,' 'useless key'). Furthermore, there's expository text lying around (notes, diaries, etc.). This stuff is displayed using the scrolling-text function used for, among other things, the opening credits, so it scrolls past in real-time at a fixed speed (plenty slow enough to read), so you need to park your ass in a quiet corner for a while, but it's a huge boost to atmosphere. Plus, the writing's flat-out hilarious. The insane key puzzle mechanic falls flat when you have no idea whatsoever where the next quest item is, which is compounded by the labyrinthian quality of these maps. It's a lot more fun when you know where to look, truthfully; some mechanics are dead and buried for a reason. Still, there's a walkthrough if you need it.
So yeah, it's worth digging out your old copy of Half-Life for, or downloading a steamless version (I don't think it plays nice with Steam, and anyway the oldschool version's a lot less obnoxious).