Engine tools
The engine tools are development tools that run within a Source engine process. They include the Particle Editor, Material Editor and Commentary Editor, and also unreleased tools like Foundry and the much-coveted Source Filmmaker.
Portal 2 does not include engine tools support, but running the game with the -tools switch does update the soundcache, which isn't done automatically for some reason.
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.
Loading new tools
Warning:You are likely to encounter instability if you try to mix engine branches.In Alien Swarm 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.