Before I started work on The Nightmare Chronicles, I though i knew just about everything there was to know about hl2 level design and the limits of what Valve's Hammer editor could do. I was wrong.
Turns out there are still a bunch of entities that i have been overlooking or not using to their full potential. The first being point_clientcommand, a little know entity that is recommended to not be used in multiplayer because it can cause problems with the server and could be exploited as a way to create a mod that harasses players. But in single player it can make use of things like valves "debug visuals" to create special effects. Do you want your mod to be entirely in black and white, just send the command "mat_yuv 1" with this entity and you've got a monochrome game without having to do any code for you mod. Now if you only want a partially black and white game, you'll need to make some grayscale textures, but thats easy. You can also do all console commands like change gravity, give weapons, spawn enemies and vehicles. Anything that there is a console command for.
Another useful tool is the env_screenoverlay, this is great for creating temporary visual effects. You can do the same black and white effect from the point clientcommand entity with this by using "debug/yuv" as the overlay texture. Or you can use a number of other neat valve created shaders to warp and distort the players view. But it can also be used to create a simple gui for sections of your mod with out touching any code. I used it to create a simple healthbar for the first boss in my mod. I was able to do this because he had a standard decrease in health delt to him by each attack you do. Thus I was able to make a small set of images to overlay on the screen during the fight to represent his health.
The last Entity I'd like to mention is the game_ui. I've skipped over this entity many times when scrolling past it in the drop down menus to get to other entities i needed and never knew the power that it held. Turns out it is a way to take the players movement input and apply it via inputs to control and activate other entities. For instance I made a boat that is moved by thrusters that takes the control of the players input to allow them to drive it around. But you could use this in many ways. You could make a crazy 3d menu, using the move left and right to navigate between items and use forward and back to change values. Or use it to turn on lights and trigger sounds, the possiblities are endless.
