Developer
Template:Entity It enables various levels of developer debugging options. This includes entity diagnostic information which will be printed to the console and also at the top of the screen.
Syntax: developer <value>
Valid values are:
- 0: No debugging information (standard behavior)
- 1: Standard debugging information (extra console messages, etc.)
- 2: More debugging information, messages can appear in the top-left of the player's HUD
- 3: Same as 2, but with more console messages Todo: Does this introduce any of the debugging stuff otherwise associated with 4?
- 4: Highest(?) level of debugging information; I/O events print to console, bounding boxes can show up on objects with VPhysics errors, etc.
developer 4 adds additional on-screen info like where the clients crosshair is pointing (not in HL2/Source 2013, shows the crosshair of all the players in multiplayer) and shows hitboxes and boundboxes when collision anomalies or errors occur.
Developer mode can also be enabled via the -dev or -developer command line switches, either as a shortcut or Steam command line. 
The Left 4 Dead series relies on this switch in order to load into developer mode.
See also
Other console commands and variables that a developer may find useful include:
If you are looking for information about being a developer for Valve games, you may instead find these pages useful: