Engine Tools
Bundled with 
Source are various engine related tools that help with doing various tasks.
List
Usage
To launch a game or mod in tools mode, run a command like this:
hl2.exe -tools -nop4 -game "path to gameinfo"
Failure to specify -nop4 will lead to the engine exiting silently.
When the game has loaded you will be presented with the first tool that was loaded, probably the Actbusy one. To change to a different tool use the Tools item in the main menu.
Setting Key Bindings
to set key bindings click Edit -> Key Bindings
the Key Bindings Editor window will appear
Todo: Explain the Key Bindings Editor
Loading new tools
Warning:You are likely to encounter instability if you try to mix engine branches.In 
Left 4 Dead 2 and later you can manage tool availability with the toolload and toolunload console commands or by editing the engine's bin/enginetools.txt file. There is a GUI window for this too but it's currently broken.
In earlier engine builds the tools system is locked down, probably to ensure that we don't run any of the tools that were leaked in 2007 from Valve in the initial Team Fortress 2 beta. They can still be run by obtaining a less-than-legal copy of the beta but of course that restricts you to a very old build of the game.
Development
The SDK provides some header files at public\toolframework\ with which new editor tools can theoretically be written. There is no sample source code, however.
