Meta Box is a powerful, professional and lightweight developer toolkit to create unlimited custom meta boxes and custom fields for WordPress. Plugin provides a huge selection of fields (over 40) and many options for each type. As a result, the plugin allows you to create metafields of almost any complexity.
When basic functionality isn't enough, there are extensions. For example, they allow you to create metafields not only for posts (by default), but also for users, taxonomy items, comments or create separate settings pages. Fields can be repeated and displayed as you like: as columns, tabs or groups.