I'm one of the contributors for the Meta Box Class here on Github (link) and before I get too deep, I wanted to ask if anyone knew of a straight forward way to allow users to add additional fields "on the fly", having an "add" button with a text field to add additional fields (i.e. list items). I've seen this done a few ways in the past, but I'm all for not reinventing the wheel.
Agreed, I've seen it too, but I don't think there's any readily available API or method.
For me, I use a lick of jQuery;
Note that you might wanna tweak those selectors if you plan on having multiple 'instances' of these.
take a look at the WP alchemy metabox class. Dimas has already sorted that out w/ his have_fields_and_multi setup