What is the Developer Console and how can I use it to change game settings?

What is the developer console?

The console is a text-based input/output interface to execute commands or change global settings in a game.

How do I enable the console?

Most Valve titles

1. Launch Steam and start the Valve game for which you wish to use the console.
2. From the main menu select Options
3. Select the Keyboard tab
4. Click the Advanced button
5. Check the box next to Enable developer console (~)
6. Click Apply

Left 4 Dead and Left 4 Dead 2

1. Launch Steam and start the Left 4 Dead or Left 4 Dead 2
2. From the main menu select Options then Keyboard/Mouse
   
3. Find the option Allow Developer Console and set it to Enabled
   

Counter-Strike 2

1. Launch Steam and start Counter-Strike 2
2. From the main menu select the gear icon (Settings menu)
3. Under the Game tab in the Game section find and click the arrow next to Enable Developer Console (~) so that it says Yes
   

Half-Life: Alyx

1. Launch Steam and go to your Library
2. Right-click Half-Life: Alyx in the Library and select Properties...
3. In the General tab go to the Launch Options section and enter: -console -vconsole
4. Exit the window and launch the game
   Note: the console will only appear on your desktop screen and not in the headset display

What can I type into the developer console?

The developer console is host to a vast variety of commands. Many of these commands have prefixes that offer a hint as to what they deal with. A partial list of available commands can be found in the Valve Developer Community.