uMenu16 aims to be a simple, lightweight, easy-to-use and open source shell replacement for the 16-bit Windows 3.x operating systems, which brings by default the old trustworthy Program Manager. The whole project has been coded in Objective Pascal under Borland Delphi, and all its source code is avaible on the project's github page.
uMenu16 follows the KISS principle (Keep It Simple, Stupid), which means that it has been designed to be as minimal as possible. While its not that customizable itself, it allows you to customize the whole rest of the system.