In the past, I have usd CDE (yes, that CDE...) on my office workstation. I like it. I wanted a simple runner -- something to bind to Alt-F2 like in many other desktops, only written as a Motif application. There didn't seem to be one, so I wrote one.
There's not much to it: you run Dtrun, it gives you a window with a text box, a "Run" button, and a "Cancel" button. You enter a command to run in the text box, click "Run" / hit Enter, and Dtrun tries to run it. If it works, it works. If it doesn't, it doesn't. Oh, and it exits on "Escape".
That's about all there is to it. Check the source if you're interested -- but really it can't get much simpler...
You'll need Motif, X11 headers, and a half-decent C compiler to build this. It should be easy to build pretty much anywhere you have those -- if not, please let me know and I'll fix it.
MIT license, see LICENSE in source for details.