Good free programs for Sandbox

2D Art

• Artweaver, a 2d paint program like photoshop. It supports layers. Some texture artist like to paint on the flat 2D uv map image for textuing and other texturing usage.
  • http://www.artweaver.de/index.php?en_version

• Inkscape, a vector graphics editor
  • http://inkscape.org/

• GIMP, or the GNU Image Manipulation Program, a photo editing program that can be used for texture making.
  • http://www.gimp.org/

• Paint.net, a different alternative to Photoshop or the GIMP.
  • http://www.getpaint.net/

• NeoTextureEdit, a procedural texture generator.
  • http://neotextureedit.sourceforge.net/

• FXGen, another procedural generator.
  • http://sourceforge.net/projects/fxgen/

• Allegorithmic MaPZone, another free texture generator.
  • http://www.mapzoneeditor.com/

• Spiral Graphics Wood Workshop, a free texture creator which specializes in wood textures.
  • http://www.spiralgraphics.biz/ww_overview.htm

3D Modelling or Related

• Wings3d, a great modeler with a outstanding uv mapper.
  • http://www.wings3d.com/

• Tattoo 3D, This was my first 3D texturing program. It great for fast texturing but limited on more powerfull functions as it a older program. Free to use for non-commerical work.
  • http://www.terabit-software.co.uk/

• Blacksmith 3D, You can paint your models in realtime 3D. The "freeware" 3D painter limits your export image size to 1024 x1024 which is more than enough for Sandbox
  • http://www.blacksmith3d.com/blacksmith3D-3d-paint-software.php

• Misfit Model 3D, to add bones and animations for your NPC or other models, plus to save your model as a md3 file with the animation data. be sure to get the lastest beta version
  • http://www.misfitcode.com/misfitmodel3d/

• CharactorFx, for adding bones and animation to your models. It's a little older program but works great and saves to md2 files.
  • http://www.insanesoftware.de/

• MD3Compile, a very quick and easy converter for md2,3ds files to a md3 file. I use it for converting my static 3ds models into a md3 file plus to rebuild my 3ds model's normals.
  • http://www.quakeunity.com/file=481

• Blender, a 3D modeling program with support for animation and UV texturing with a large community behind it.
  • http://www.blender.org/

• Arbaro, a tree generator written in Java which can export Wavefront OBJ files, which are supported by Sandbox.
  • http://arbaro.sourceforge.net/

• Pixologic Sculptris, a sculpting-based 3D modelling program.
  • http://www.pixologic.com/sculptris/

Other

• Notepad Plus (++), unique text tool built mainly for scripting of many different types. This can be used to edit/make CFGs in Sandbox.
  • http://notepad-plus.sourceforge.net/uk/site.htm

• Audacity, a program useful for editing audio or converting audio files such as mp3 wav ogg
  • http://audacity.sourceforge.net/download/

• Linux Multimedia Studio, a Linux AND Windows music making program.
  • http://lmms.sourceforge.net/

For many more free programs, look at http://sourceforge.net/ and http://portableapps.com/