Here is everything out to get you in Half-Life 2

Name Description Picture
Headcrab - Slow The Headcrabs we all know and hate from Half-Life 1 and it's add-ons. These will jump at you and try to attach to your head.
Headcrab - Fast These will do the same as the first, but are much faster and look different.
Headcrab - Nasty These nasty ones will jump at your head and make you go to one health straight away by injecting Neurotoxin. But don't worry too much about that, because the hazard suit will heal you back to your previous health level.
Zombie - Slow This is what happens if one of the slow Headcrabs attaches to your head. They are very slow, but will lash out at you if you get too close. They will throw anything at you that is in their path. The Headcrab is able to detach itself if you destroy the body so be careful.
Zombie - Fast This is what becomes of a human who gets attacked by a fast Headcrab. They will climb pipes and attack you at high speed.
Zombie - Nasty These are humans that have been taken over by nasty Headcrabs. They have three or four nasty Headcrabs on them that they will throw at you.
Barnacle The same old barnacles from the old game, with an added mouth inside. If you don't know, they will pull you up with their tongues and eat you, so don't walk into them where possible.
Ant Lions Make sure you don't walk on sand - otherwise about five of these things will be summoned. They can fly, but aren't that difficult to kill, just try not to walk on sand in the first place where possible. You won't see them till Highway 17, then you have to be careful. Once you get some Pheropods (bugbait) however, they'll become some helpful allies. Ant Lions
Mother Ant Lion A sort of boss you'll encounter in Sand Traps and Nova Prospekt. Quite difficult to kill, but with a few rockets, grenades and lots of shooting, you'll finish it off.

Humans (or at least things that look like Humans) and Robots
Name Description Picture
Police Basically evil policeman who will hurt anything for whatever the reason. They normally carry either  9mm Pistols or electrified beating sticks, although they can be found with SMG (Sub-Machine Guns) on later levels.
Combine Soldiers These soldiers guard City 17 and shoot you and members of The Rebellion until you die. They carry a lot of different weapons, and depending on the weapons they're carrying, can be easy or reasonably difficult to kill.
High Security Combine Soldiers Slightly harder to kill than normal Combine Soldiers because they have better armour and always carry an Overwatch Standard Issue (Pulse-Rifle) and will use the secondary mode.
Striders Giant things on legs that have a powerful machine gun and take about five rockets to destroy. There's normally an unlimited rocket box around when you need to destroy one of these.
Combine Helicopters They shoot at you, and take three rockets to destroy (on easy, more on harder settings). Again, you'll normally find an unlimited rocket box around when you need to destroy one of these.  
Floating camera Things These things take your picture - great! Well no, it tells the combine where you are, so destroy it before it gets your whereabouts known.
Man Hacks These little robots - sent by the combine - flood in and hit you with their blades, so shoot them quick.
Dr. Breen Your old administrator back at Black Mesa, he seems to be behind everything that happens in City 17