Engine tools
The engine tools are development tools that run within a Source engine instance. They are accessed by running any post-Orange Box game with the -tools parameter. They include the Particle Editor, Material Editor and Commentary Editor, and also unreleased tools like Foundry and the much-coveted Source Filmmaker.
Usage
The engine tools system is currently locked down. The enginetools.txt file that usually defines which tools are active is not read, nor is the sdkenginetools.txt file seen in Source SDK Base 2007. It is also impossible to run in tools mode when launching hl2.exe directly, probably to ensure that the Steam "launching game" process, which overwrites any customised files in the engine folder, is not bypassed. Even if you do bypass it (by using a script which copies the custom files back over Valve's in between Steam finishing its launch and the engine reading them) the replacement file is rejected.
This effort is apparently all to ensure we don't run any of the tools that were leaked from Valve in the initial Team Fortress 2 beta from 2007. They can still be run by obtaining a less-than-legal copy of the beta, but of course that gives you a very old, outdated build of the game.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, and as explained above it's not currently thought possible to make the engine load your library even if it is properly formed!