Valve has provided Authoring Tools for each of their games. Left 4 Dead 2 itself is a cross-platform game, but the authoring toolkit is not. However, we can run it through Wine or CrossOver.

After enabling Steam Play, we can download Left 4 Dead 2 Authoring Tools, inside which the Workshop Manager can be launched directly. For the other 3 tools: Face Poser, Model Viewer, Hammer World Editor, we can find their Windows Executive files in directory $GameRoot/bin, and launch through Wine.

VPK is Valve's file format for storing game assets. ValvePython/vpk[github.com] provides a Pythonic access to VPK files and their contents together with a cli tool.

Unvpk[github.com] provides facilities to list, check, extract, analyze and mount VPK archives. You can get the source and build it yourself.

GNOME Archive Manager with Valve Pak(vpk) support. file-roller-vpk[github.com]

The Valve Texture Format (VTF) is the proprietary texture format used by the Source engine.
gimp-plugin-vtf[github.com] is a plugin to open and save vtf files in GIMP.

The KeyValues Format(VDF) is used in the Source engine to store meta data for resources, scripts, materials, VGUI elements, and more. ValvePython/vdf[github.com] is a pure Python module for (de)serialization to and from VDF that works just like JSON.

eXeC64/vtf2png[github.com] is a command line tool to extract images from the VTF format used by Valve's Source Engine. It supports a variety of pixel data formats, including most of the RGBA variations and DXT compression formats.

(Optional)gdk-pixbuf-vtf[github.com] is a GdkPixbuf image loader, which lets you browse VTF files in Nautilus or open them with Eog, letting you export them.

Vice is Valve's tool for ICE encryption, a trick used mainly to protect script files from modification. Download the pre-built binaries or source code from Valve Developer Community - Using Vice without Steam