MBuilder: A front end compiling tool to compile your maps. Simple and easy to use.
PakScape: A simple program that lets you create/manage .pk3 files.
Radiant Plugin: Lets you purge evil polys/duplicate planes, and also has a built-in stairmaker. DDS Plugin: (For Adobe Photoshop) This plugin will let you open and edit dds files.
TGA Previewer:(Windows Only) This plugin will let you see a preview thumbnail of a tga file without having to open it.