GUI for Standard ML?

There are bindings for foreign toolkits, but the eXene toolkit was designed to be native for SML and to exploit the features of Concurrent ML. I used it years ago and found it a very smooth fit for the language and a pleasure to use. But it does not have the ginormous library of components that you find in the more widely used toolkits.

Here's a paper describing the mGTK project, which is a Standard ML binding to GTK. There are also a couple of interfaces to OpenGL, although this is more 2 and 3D graphics-oriented rather than specifically a GUI toolkit. eXene is a toolkit for X Windows. sml_tk is an interface to Tk.