How to create a plugin that notifies for updates?

Question

At this point when I activate a plugin and an update is available it updates the plugin. I like to create a plugin myself that I want to use private, I have several WordPress blogs and if I made an update to the plugin I like those several WordPress blogs to state that there is an update available. Does anyone know how to do that?

Answer 1

There is a plugin for that: Automatic Updates For Private And Commercial Plugins.

To prevent upgrade checks against the WordPress.org data base in your plugin use the function Mark Jaquith has written.