Programming language for writing simple GUIs
I wrote a programming language called Sindre. It came about after I got tired of writing simple X11 tools directly against the Xlib API. At the time I was doing a lot of shell scripting, and I had become enamored with the pattern-action mode of programming found in AWK, and I realised I could implement a similar language where the patterns were events (both from X11 and in the MVC sense), and the actions were… well, imperative code. Combined with a simple way to declaratively specify the GUI, I would have all the functionality I needed to write simple programs to interact with the likes of the surf browser.
I wrote a number of small tools, including controlling programs for surf (basically just a more intelligent dmenu), and gsmenu, the latter of which I still use for managing windows in Xmonad.
Unfortunately, I have more or less given up on the idea of loosely composable GUI programs. It is simply too difficult to pass around the necessary information, and the programs written to support it (like surf) tend to lack important features. For example, surf is lacking in security, key management, stability, and many other small quality-of-life features.
But I suppose that given I still use programs written in Sindre every day, it has been a success of a sort.