A few of the open-source ones:
https://garage.maemo.org/projects/adv-backlight/ (statusbar applet)
https://garage.maemo.org/projects/personal-launch/ (home applet)
https://garage.maemo.org/projects/personal-menu/ (navigator applet)

There's also hello-world applets for all 3, but I think I've heard they're somewhat lacking, so starting with completed example might be more helpful.