Starsiege TRIBES Players site has the release of the development tools for Starsiege TRIBES on their Multimedia and Downloads page. Thanks [Inq]Colosus. Here's the skinny on the release from the page:

TRIBES Tool Kit
Below is the link to download the TRIBES tool kit. Please be aware that this is a non-supported release; in other words, we give you these tools "as is", use at your own risk. This is by no means a required download, that is, this is not a patch. It's a collection of tools and utilities that we used to create objects and missions for TRIBES. If you aren't interested in creating your own missions, or customizing the graphics in the game, don't bother downloading this. For a listing of the tools included in this package, along with short descriptions of their functions, please examine the readme file before downloading.