You are looking for Repeatable Fields.
Here in WPSE, it was covered in the following Q&A:
Custom Meta Boxes: Store two values in one repeatable field
A Gist:
https://gist.github.com/2057532
Class helpers for building the plugin options page:
- Option Panel Class (from Bainternet, one of the house moderators)
- WPAlchemy_MetaBox PHP class
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