Starting in WordPress 2.7, the Dashboard is totally customizable. You can drag the Dashboard widgets (not to be confused with sidebar widgets) around, and hide/show them.
Now what if you want to add your own widgets? Maybe your plugin could use one, or you simply want to use the functions.php file to put a custom widget on your own blog.
There is a simple function, and accompanying hook, allowing you to add widgets to the Dashboard. It’s not exactly easy to find documentation for the functions, but it exists. You just need to look in the right place (in the WordPress Codex, of all places…).
There you will find a nice overview and some code samples. Have fun.