I just published another plugin called Simple Modal: http://www.apex-plugin.com/oracle-apex-plugins/dynamic-action-plugin/simple-modal.html
This plugin allows you to use any region(s) (or DOM object) in your APEX application as a modal window.
When developing this plugin I learned a few more things that may be useful when developing plugins:
Scope Creep: When developing a plugin you can make it do a lot of things. This may lead you to try and include extra unnecessary functionality. Try to remember the goal you're trying to achieve, or more importantly, what you're developers will try to achieve with the plugin.