• Paint.NET - Far superior to GIMP and Photoshop; great for creating textures for source games. Download effect plugin packs here and file type plugins here. Eliminate the use of VTFEdit by downloading the PDN VTF plugin here. There is also a normal map plugin available here.
• Audacity - A superior, free sound editor.
• Wavosaur - Another sound editor. I use this to loop sounds for custom soundscapes. No installation option; just unrar and launch.
• VIDE - Packs custom content into BSP files.
• GCFScape - Extract assets (sounds, soundscapes, VMTs, textures, etc.) from Source games. Useful for map development.
• Notepad++ - An enhanced version of Notepad. Useful for scripting.
• WinRAR - Used to extract packed files. Download rarreg.key to disable the annoying popup telling you to buy the program. Drag it into the main program directory and confirm (or replace).