Custom Meta Boxes is a framework for easily adding custom fields to the WordPress post edit page.
It includes several field types including WYSIWYG, media upload and dates (see wiki for a full list). It also supports repeatable and grouped fields.
This project is aimed at developers and is easily extended and customised. It takes a highly modular, Object Orientated approach, with each field as an extension of the CMB_Field abstract class.
The framework also features a basic layout engine for fields, allowing you to align fields to a simple 12 column grid.