I guess the most used metabox library that I have used is Custom Metaboxes and Fields for WordPress written by Jared Atchison. The good part about this library is that it's very much like plug-and-play. Just include it in your theme, fill in all the required fields and you're done. The creator of the library is a great coder and his work is really amazing. The code quality is brilliant and he even includes a sample file which you can use easily (if ever you get confused).
Another great resource for creating metaboxes is WPAlchemy by Far In Space. Now what sets apart WPAlchemy from everything else out there is the super extensive documentation and the wide range of features available.
One of them is called Meta Box.