ButterBean is a new drop-in custom fields suite by Justin Tadlock
ButterBean supports a number of useful field types out of the box, and is extensible so others can create more field types of their own. Syntactically, it’s pretty similar to working with fields in the customizer, which it’s modeled on.
The interface itself is similar to WooCommerce’s tabbed product metabox. I think this sort of interface works well for WordPress metadata. Since this project is a drop-in, it’s designed to be bundled inside of plugins, so you can save yourself some time using this if you need some basic field types. Just originally started ButterBean for use in his portfolio plugin.
I look forward to trying Justin’s project on a real life website, but I already know it’ll feel like I’m right at home. I’ve used his code for a long time — going back before he released his Hybrid Core framework — and it’s always been a pleasure.
The 1.0 release does not include repeatable fields, but he tells me it’s on the agenda. It currently supports any post type, and Justin isn’t planning to enable support for user or term meta. He says he’ll consider it if WordPress core ever decides to support meta boxes on those screens.
More information for ButterBean is on the project page, it is available to fork on Github, and there’s a sample plugin for using it too.