Good free programs for Sandbox
All the below programs are free:
Wings3d, a great modeler with a outstanding uv mapper.
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
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
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.
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
Notepad Plus (++), unique text tool built mainly for scripting of many different types. This can be used to edit/make CFGs in Sandbox.