What backend-agnostic admin interfaces / GUIs are there?

This is a community wiki answer. You should edit me with your improvements. Alphabetically sorted.

Implementations

AdminJS is a database table like GUI with CRUD abilities
NOTE: No longer maintained.

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Create.js is an inline editing GUI for editing content and managing collections, it provides a widget interface and can be extended to have WYSIWYG editors placed ontop of it

_{(source: bergie.iki.fi)}
HydraConsole is a generic API console for Hydra-powered Web APIs.
Lumber Lumber uses your SQL or MongoDB database to generates a modern Admin Interface application that is completely backend agnostic and highly flexible.

ng-admin: Add an AngularJS admin GUI to any RESTful API
Web Write's InlineGUI is an inline editing GUI with CRUD and site management abilities

Create.js Backend Integration Guide - REST API spec for CRUD actions
VIE - server-agnostic JavaScript data layer built on the W3C RDFa and JSON-LD specs and used in Create.js
Hydra - a lightweight vocabulary to create hypermedia-driven Web APIs. By specifying a number of concepts commonly used in Web APIs it enables the creation of generic API clients.
Web Write Spec - REST API spec for CRUD actions and collection listings
Neutron Protocol - XML and WebDAV protocol for content editing
The Atom Publishing Protocol - XML and WebDAV protocol for content editing
MetaWeblog API - XML-RPC API for blog-oriented content editing
Data.js - data representation framework for Javascript
Twingle - XUL application and protocol for cross-system content editing (discontinued)

There is also https://github.com/punkave/jquery-rest-admin In this context http://jsonapi.org could also start playing a role.