Upspring
From Spring

Upspring
Upspring is a Model Converter/Editor for the Spring RTS Engine.
It can load and save .s3o .3do .3ds .obj .opk .c3o files, the first two of which are used by the Spring engine.
Download
Win32 installer:
Source Code:
Common Issues
http://answers.springlobby.info/questions/690/why-does-upspring-mirror-my-model-when-i-import
http://answers.springlobby.info/questions/398/upspring-how-to-set-piece-origin