Half-Life Cheats

A technique used in a lot of Ego-Shooters is the Rocket-Jump where you point a Rocket Launcher at the ground and fire off a rocket while jumping to be able to jump way higher than usually. The downside is that you lose health by doing this and since a rocket in Half-Life takes away lots of health it wouldn't really be of good use.
Instead you can exploit another possibility in Half-Life: Using the secondary fire mode of the Gauss Cannon instead of the rocket launcher will also launch you high into the air but will not directly deal any damage (only eventual falling damage).

Use a Snark (a bug) when facing a wall and jump onto it. Directly throw another one and repeat until you've reached the height you wanted to reach. This way you are able to reach places you're not supposed to reach just yet or even places that aren't supposed to ever be reached like roofs that can be used as shortcuts.

Put Trip Mines onto a wall in form of a small staircase. You can actually jump on them and even use them to reach places you're not supposed to reach just yet or even places that aren't supposed to ever be reached like roofs that can be used as shortcuts.