This will give you information about when you're damaged, and from which direction it comes. If you're damaged from all directions, it'll look like the picture.
Acid or Corrosive Chemicals
This stuff is very, very rare, but it hurts pretty much, so stay out of it.
You'll start to loose energy if you stay under water for more than 12 seconds. You'll then loose 5 health / sec., and when you get some air, you'll regain 10 health / 2 sec. of what you've lost.
There are several things that can give you electrical shock, such as power wires hanging down, Slaves and malfunctioning power generators. It generally does a fair amount of damage.
Mostly comes from broken pipes, and it can kill you fast if you don't get out of it in a hurry. The Garg also has two flame-throwers which can kill you in a snap.
Extremely, insanely rare (found once), and this won't hurt much unless you stay in it for long periods of time.
Often found in the Black Mesa complex, this will damage you even if you're not in direct contact with it. It'll keep hurting you by 5 health points after you've jumped out of it.
Also mostly found at BM, you'll hear your Geiger counter tick faster and faster as you approach it. Strangely enough, you have to be in direct contact with the stuff to get hurt.
Explosives can be very useful for killing enemies. Just shoot the explosive when an enemy is near, and they'll often be gibbed.
Often found at laboratories.
Rare, but highly explosive.
Liquid Nitrogen Container
Found a couple of times. Extremely explosive.
Crate Containing Dynamite
Found in the middle sections of the game.
Crate Containing Explosive Chemicals
Found in the later sections of the game.
These things (shown as already exploded) are pretty rare, you won't be able to see them since they're hidden underneath the ground. They'll explode if you step on them, or if you hit them with some kind of projectile. If you see a sign that says "Mines", throw a hand grenade into the field, and you'll probably set off a chain of explosions. Any projectile will set them off also, so try using the MP5.
These are found nearly everywhere, and come in all sizes. Some of them only need one hit from crowbar, some need more. They can also provide cover for you in combat situations. You should always break open every crate you see, since they often contain ammo or health. Sometimes, though, they are needed to get somewhere, so think twice before you do something stupid.
There are two kinds of these, and they practically do the same. When you stand in them, you'll be slowly healed, 1 health point / 0.5 sec. for the tall one (right), and 2 health / 0.53 sec. in the pool (left).
You'll find them only in the Alien World, where they'll vital for your survival as there isn't much health otherwise. They can also appear in the form of large pools, where you can literally wade around and regain health.
Guns You Can Control
One of the absolute coolest things about Half-Life is the interactive environment. There are tons of different weapons you can take control over simply by "use-buttoning" them. They range from heave machine guns, to guns that fire artillery shells that can destroy almost anything in a single shot.
Jump onto these, and use them. You can now accelerate or decelerate the train with the forward and backward keys. They can go very fast, and can smash through wood for instance. Watch out that you don't get in contact with the middle rail (yellow/black), or you'll be hurt.
You'll need these to perform high jumps. Just step on to them, and you'll be thrown into the air. Watch out that you don't hit the ground from far heights or you'll be seriously hurt.
Only found on the "Lambda Core" level, these things are top secret, and highly experimental. If you walk into the one shown on the left side of the pic, you'll come out of one shown to the right, nearby. In the Alien World, you'll also find teleports that take you from one level to another.
You'll only bump into these on one level in the game. Out from them come colored beams, and the pod will break if something comes in the way of the beams or if you shoot at it. Five Snarks will then come out of it, and chase you until they explode after 15 seconds. There's no way to avoid breaking the pods on that level, so just run back as soon as you break it, and shoot the Snarks with the MP5.
Only found on one level in the entire game, these things will start to rumble at bit if you fall into them, and then send you towards the sky like a rocket. I'm not joking when I say about 200 feet upwards. You will be killed if you hit the ground from that height, so.....well, just enjoy the view for a moment and load your last saved game in case you fall into it.
Also only found on one level, you'll need to jump onto one of these to get to a certain teleport at that level. They're rock solid, so you can't kill them.
These things will sometimes sweep across land, and deploy an alien on the ground. They can't be killed.
You will find these a couple of places in the Alien World. They provide light for you, but if you get too close, they'll stop glowing until you walk away again.
There are a lot of these at Black Mesa. Some of them have buttons you can push, do that, and a soda will pop out! If you stand right in front of it and use it like that, you'll gain 1 health point, you can do it up to 10 times.
Crate With Alien Grunt Inside
If you've never thought of shooting at one of these, don't start, because they contain an Alien Grunt that's not nice to meet. They only need a couple of rounds from the MP5 for example to break.
Found all over the Alien World. There will often be a backpack next to them.