Take a good look at the enemies you'll see in Half-Life 2 as they'll all be nothing but blood and guts splattered on the floor once you've finished with them.
 

Classic Headcrab
		First Seen: Chapter Three, Route Kanal The most common alien in the game. These little blighters just love hiding…well…everywhere, but you can expect to find them in any dark corners, crevices, or the heads of unfortunate passers-by. Easy enough to despatch with a well timed swing of the crowbar or a couple of handgun rounds, but don't underestimate them. They can, and frequently do, leap at you from a distance attempting to latch onto your face and have a nibble on that tasty brain matter. You can often hear them coming though, keep your ears peeled for small clicking noises and quiet screams.
Fast Headcrab
		First Seen: Chapter Six, "We don't go to Ravenholm…" Just when you think you've got the hang of whacking normal headcrabs for six, you're encountered with this beast. These headcrabs are just like any other headcrab except they can run faster, jump higher and do slightly more damage. Getting into a tangle with a pack of these can be tricky so if possible try and get them from afar with a few well placed pistol shots. Just like the normal headcrabs, their main goal is to attach to your face and make you into a zombie. If you meet this fate, it won't be pleasant.
Black Headcrab
		First Seen: Chapter Six, "We don't go to Ravenholm…" Ok, so you've mastered the fine art of headcrab whackin' with the crowbar, learnt how to deal with the fast little buggers and then they pull out the big guns. The black headcrabs are skinnier, darker, uglier and a tad more deadly. They contain a deadly poison that doesn't actually kill you, but it will take you all the way down to only having 1 health point left. Not only do you have to deal with the threat of one latching onto your face and turning you into the living dead, but also of an alien venom coursing through your veins.
Classic Zombie
		First Seen: Chapter Three, Route Kanal This is what happens when you let a headcrab suck out your brains. These shambling shells of former human beings wander the streets looking for fresh meat, guided only by primal instincts and that pulsating cream blob with legs sat where the head used to be. Their basic attack is a swipe from the their deadly claw-like hands, another side-effect of the headcrab infection. The best defence is a couple of rounds to the head to take out the headcrab parasite, as if the host body takes too much damage before the crab dies it can detach and leap at you itself.
Fast Zombie
		First Seen: Chapter Six, "We don't go to Ravenholm…" Zombies are slow, right? That's their major disadvantage, right? They can't close the gap to get in a close range swipe before you fill them with lead, right? Wrong. This new zombie variant is the most deadly yet. Capable of insane speeds and an even more ferocious attack, they will eviscerate you in a blink of a Vortigaunt's singular red eye. These mercurial monsters like to haunt old graveyards and woods, leaping at their unsuspecting prey from trees and bushes. And as if that wasn't enough, they often hunt in packs, so you need to keep your wits about you when trekking through their territory.
Posion Zombie
		First Seen: Chapter Six, "We don't go to Ravenholm…" The next step up from the fast zombie isn't actually very fast at all. Instead he's really slow and part of the reason for that is he has a number of poison headcrabs controlling his movement. The dangerous thing about these is they have a tendency to launch poison headcrabs at you. These guys aren't pleasant, especially when you're already being attacked from all sides by other nasty creatures. These guys are as tough as nails too, it'll take a long time to take them down.
Barnacle
		First Seen: Chapter Three, Route Kanal The reason you should always check to see what that handy rope is attached to. Barnacles hang around on ceilings dangling their tongues down waiting for it's next meal. The tongue isn't its main weapon though, that title goes to the pair of super strong jaws lined with razor sharp teeth at the top. It'll bide it's time, waiting until your back is turned and then it'll whip it's tongue round your neck and drag you up to a horrible death. It's not all bad though, the fact that they are essentially immobile means that if spotted early enough you can take them out before they can reach you.
Ant Lion
		First Seen: Chapter Seven, Highway 17 Just one of the new aliens we stumble across on our travels through City17. Although a deadly foe, with the use of Alien Pheromones you can trick these beasts into believing you're one of them and then use them to attack your other enemies within the game. They're very subservient in that respect but make sure you run when their real mother makes an appearance. They might not be extremely clever, but their loyalties won't lie with you when the Ant Lion Guard comes looking for them!
Ant Lion Guard
		First Seen: Chapter Eight, Sandtraps Three times as big and three times as strong. Get too close to one of these aliens and you'll find yourself flying through the air. These get incredibly grumpy, especially when their smaller Ant Lions appear to be in danger. Parts of their bodies are armour plated and explosives are usually the best way to dispatch them, but even that can take more than a couple of rockets…
Civil Protection (Metro Cop)
		First Seen: Chapter One, Point Insertion The Combine Civil Protection Unit's "Metro Cops". Early on in the game these will be equipped solely with a stun baton for keeping you in check if you're causing trouble. Later on, they're not so fun when they arm themselves with hand guns and automatic weapons. It's usually best to take these guys out with a few shots from a handgun, but blowing them up and hurtling them across the city is usually much more entertaining.
Combine Soldier
		First Seen: Chapter Six, "We don't go to Ravenholm…" Former humans, vastly upgraded with Combine technology, the Overwatch soldiers prove tricky adversaries. Well armed, with either the SMG, Shotgun or pulse rifle, and with a tendency to throw grenades in you’re direction, Combine soldiers are able to utilize a variety of tactics, and are often found to work in squads.
Combine Elite
		First Seen: Chapter Nine, Nova Prospekt Vastly superior to the Combine soldier, the Combine Elite is the next stage of the Overwatch forces. Armed with a Pulse rifle, the Elite exceeds the soldier in terms of armour, strength and sheer force. Often seen leading squads of Combine soldiers, the Elite operate as the pinnacle of the Overwatch forces.
Combine Strider
		First Seen: Chapter One, Point Insertion A Combine Synth, these monstrous tripods are only deployed when Overwatch forces are stretched thin. Armed with a rapid-fire turret on its snout, and the ability to use the ‘pulse cannon’, the Strider provides a serious threat to resistance forces. Resilient to most weapons, your only chance of taking down a Strider would be the rocket launcher.
Combine Gunship
		Chapter One, Point Insertion Another Synth, the Gunship is the cream of the crop in the Combines air forces. Armed with a rapid pulse cannon the Gunship is capable of swiftly dealing with on-foot resistance. The only weapon with enough force to take down this assault craft is the RPG, however the Gunship is very much aware of this and will attempt to shoot down your rocket.
Combine Dropship
		First Seen: Chapter Four, Water Hazard This Synth aircraft is designed to carry cargo to and from the Citadel. Carrying payloads such as troop containers, Striders and APC’s. You can’t actually take these down, so your best bet is to go for whatever it is carrying. It’s also worth noting that the troop containers possess pulse cannons.
Scanner Type I
		First Seen: Chapter One, Point Insertion These scanners patrol the streets of the City in vast numbers, relaying anything they find to Civil Protection monitors. More of a nuisance than a threat, their only form of attack is a spotlight which will temporarily blind you. Use the crowbar to take these bothersome Scanners down.
Scanner Type II
		First Seen: Chapter Eleven, Anticitizen One A more Synth like entity than the previous scanner type, it’s armed with a fast firing machine gun and often carries ‘Hoppers’ into the battlefield. Easy to take down and far from a dangerous threat.
Rollermines
		First Seen: Chapter Seven, Highway 17 Deployed mainly in the areas surrounding City 17, these 'Rollermines' can sap your suit power in a matter of moments, using their electrical attack to knock you back. They can also latch themselves onto the buggy. The best defence against these pests is to pick them up with the Gravity gun and throw them into the sea.
Manhack
		First Seen: Chapter Three, Route Kanal The name seems to shed some light on what these robots will do if they hit you. A few well aimed gun shots or a couple of whacks with your trusty crowbar should send them to silicon heaven. Again, this is another one you can use the manipulator on. Why not pick it up and use it against other enemies in the game as a sort of make-shift chainsaw?
Hunter Chopper
		First Seen: Chapter Three, Route Kanal A new Combine attack helicopter built upon new human technology. The helicopters are loaded with mines and a gun turret to take you out and they're able to do a lot of damage to you. If you have one available, use the RPG launcher, otherwise you'll have to try and use other weapons you have at your disposal to help take these down.
Armoured Personnel Carrier (APC)
		First Seen: Chapter One, Point Insertion The Combine Armoured personnel carriers are not often found in the combat zone, mainly used to ferry Combine soldiers from one location to another. Heavily armoured and carrying pulse cannons, your only chance of taking these down would be either rockets or the auto-cannon equipped to the Airboat.