Designed for use in embedded devices the mGSTEP libraries implement a subset of NeXT's OPENSTEP specification which has evolved into Apple's Cocoa Objective-C frameworks. The internal graphics rasterization supports rendering to an X11 or Linux frame-buffer based GUI.
Baby X is a simple toolkit for X Windows.
There's also the Baby X resource compiler, which can be used as a standalone program or to get resources together for Baby X programs.