SoXt is an Xt/Motif glue library for Coin. It can also be used on top of the SGI or TGS implementation of Open Inventor, and is designed to be source code compatible with SGI's InventorXt library.
You will need either Motif (Open Motif) or Lesstif installed on your system to use SoXt.
SoXt is still in alpha mode, meaning that it is buggy and lacks functionality. In other words, hope for the best but expect the worst.
The corresponding documentation for Coin is located here.