Dave Gnukem is a 2D scrolling platform game similar to, and inspired by Duke Nukem 1.
The program started life in 1994 and didn't see any significant development between 2004-2016, so it's no surprise that it's a little basic.
How basic? The game help describes the plot as "Shoot anything that moves".
Still, there are plenty of plus points. The package is compact (<7MB). Doesn't need installation. Doesn't have any stupid dependencies. Runs on the most ancient of PCs up to Windows 10 systems.
It's also good fun to play, if you like this kind of thing, and has the supporting features you'd expect (redefine keys, save/ resume game, high scores etc).
2018/04/03 - Version
[3 Apr 2018] Change it so that you must first turn cheat/debugging stuff (Ctrl+Shift+G) on before using F4 or F5 for sprite/level editor
[1 Apr 2018] Add Shift+F10 to start/stop auto-save screenshot-per-frame sequence (warning, eats disk space very fast)