Leafpad is a very simple GTK+ text editor. Maybe its kindle port will be useful to someone. It works with firmwares 5.1.x–5.3.x. It is also reported to work on Paperwhite. I can't test it on other devices.
I prepared package to be used with YiFanLu's GUI Launcher or Unified Applications Launcher (for firmware 5.3 and higher). First install the Launcher, then download zip package from this page and unzip its contents to extensions folder on your kindle. Restart kindle, so that new item is visible in the Launcher menu. Start it from the menu.
Installation of this package doesn't change any system files on your Kindle. All the files are installed only in extensions folder. To uninstall just delete leafpad folder from extensions.