Simplemusic, as the name suggests, is a very simple plugin that plays music files. It exposes a simplemusic object that you can use in scripts, and uses the SDL_mixer library to do the playing (thus it supports MP3, OGG, MOD etc.). It also demonstrates how easy it is to make a plugin. See simplemusic.txt and simplemusic.c in plugins/simplemusic for more information.
Filebrowser is another simple plugin that operates in conjunction with a listbox widget in order to display a list of files for the user to select from. Filebrowser automates the listbox so that selecting a directory browses into that directory, and selecting a file generates an event. See filebrowser.txt and filebrowser.c in plugins/filebrowser for more information.
The testbench plugin is a good example of creating widgets at runtime, and also serves to explore the properties available for various different types of widget. This plugin is available for download separately from the eboxy homepage.