Adminer offers a customization feature since version 2. It is very powerful but users can find difficult to create them. This is the main reason why plugins were published. They are simple reusable components enhancing or altering the behavior of Adminer.
There are currently plugins for XML export, HTML editor, uploading to filesystem and many more. You can easily create a new plugin and there is already also some user-contributed content.
The plugins are immediately available even for older versions of Adminer (most of them would work since version 2).