Mupload provides a file field (and image field) widget that wraps
a Plupload form element. This means multiple file uploads across
a wide range of browsers. It even supports drag-and-drop.
For a file field (or image field) use the Muploader widget instead of
the default File (or Image) widget.
This module is for Drupal 7 only!
Requirements
A fully configured Plupload module. The developer version is recommended.
Install the module as usual. Nothing will happen, but if you have a file or image field on one of your content types, now you can choose the "Muploader" widget. This will replace the standard one-file-at-a-time button with a fully operational Plupload widget.
Credits
Credits goes to czigor for his "Multiupload Filefield Widget", without
which I would have spend a lot more time figuring out the Drupal 7
file field mechanics.