Half-Life

If you hear vortigaunts spawning in, run for cover, their beam attack almost never misses if you're out in the open. But they take a while to charge and don't really move about so you can easily pop out and pop them between attacks. (Another option is to rush into melee ranger and use a shotgun, their melee attack does far less damage than their ranged lighting one.)

If a barnacle grabs you, wait until it's almost pulled you up to it then crowbar it in the face. Much easier than wasting ammo. (Only advisable if it's a short distance off the ground, otherwise when it dies, so do you.)

Use the "swap to previous weapon" button because the weapon selection screen's kind of a pain until you get used to it.

The game's rocket jump is that laser gun that charges up. It's called the Gauss, the charged up beam can go through walls and will launch you if you jump then fire it at the ground.

Some guns share ammo, most don't. I think the glock and SMG do, and the gauss and gluon gun do.

Opposing Force

The barnacle gun hooks on to biological targets only, and it's fun but unreliable against enemies.

The teleport gun will teleport anyone it hits directly, but the splash damage from hitting the wall next to them is usually a kill. Secondary fire teleports you to another area, which is different depending where you use it and leads to some Easter eggs. There will be a different area in between every "loading..." section. Most will actually be one of three generic rooms, but some will be actually be unique. Just give it a try every time the game loads a new level.